Posted by: splillo | April 29, 2012

Where did summer go?

For the last week it would seem that the Northeast has been tossed into a new ice age. Today marks the longest stretch of below normal temperatures in the region since the end of October into early November last year (coinciding with the historic snowstorm).

High temperatures have generally been held to the 50's with lows dipping into the 30's and even 20's. For the third night in a row, frost and freeze warnings have been issued across southern New York, Pennsylvania, and into southern New England where the growing season has officially begun. Snowflakes were even seen whipping through the air in many areas of New England Friday night. Still, this stretch of cooler weather is no extraordinary occurrence. In fact, we're only averaging a few degrees below normal. It's just that in comparison to the record-smashing warmth, this feels like the winter we never had.

Following the significant rainstorm last week, an upper level trough has permanently set up shop over the region, associated with cold and windy conditions. This trough will finally kick eastward tomorrow, with sunny skies prevailing but calmer winds, and warmer temperatures. Monday looks like the pick of the week at this point. Though more ridging builds to our south, the jet stream is still riding over us, carrying disturbances along the way. The threat for showers will be focused on Tuesday and Thursday, though cloudy skies stick around for Wednesday and Friday too. Temperatures will be back around normal though, generally in the 60's to near 70 if the sun should break out.

Elsewhere around the United States, the east coast of Florida is keeping an eye on a disturbance that has been spinning over the Bahamas for the last day or so. Notice the streaks of green and blue on the satellite image (courtesy of rap.ucar.edu). This is cirrus outflow indicating a level of organization and singularity to the storm. The circulation is still primarily in the mid to upper levels (versus a surface circulation that defines a tropical system), though there are some subtropical characteristics. The distinction matters little in terms of sensible weather however. The amount of moisture being pumped toward eastern Florida is near record levels for the time of year. If this storm continues to sit in place, rainfall totals of 5 to 10 inches are possible, with considerable flooding concerns.

Posted by: splillo | April 23, 2012

Major coastal storm

As I suggested in the last post, a drought-busting storm is in progress. There are two components that are coming together to form this monster. The first is a disturbance in the Gulf of Mexico that is pumping moisture northward. The second is a significant trough digging into the Ohio Valley. This trough is responsible for driving a cold front through the Northeast Saturday night which brought a round of showers and a few thunderstorms to the region. The southern disturbance then started to get dragged up the coast ahead of the trough. Ultimately, the two systems joined forces to produce a powerful moist southeast flow off the Atlantic Ocean.

The front that crossed us Saturday night stalled across the region as the southeast flow increased, and this provided a focus for heavy rain Sunday afternoon, overnight and into today. Widespread rainfall totals around 2-4" were reported from New Jersey and Long Island up into southern New England (see the map on the right courtesy of the Northeast River Forecast Center) . Low pressure took a generic track up the coast toward the Delmarva Peninsula, but then as the whole system got wrapped up, the low turned more northward. It currently sits over New York City, and will track inland over New York state this afternoon. To the west of the low, there is another side of this storm: SNOW! Western Pennsylvania and New York are facing a major spring snowstorm as bands rotate around the large circulation into the cold sector. Accumulations of 8 to 16" are possible through tonight.

So here's a fun thought: the two biggest snowstorms for the Northeast this season were October 29th and April 23rd! And in between, temperatures soared into the 70's and 80's on several days. So much for climatology!

Looking ahead, this storm establishes a trough over the Northeast at least through next weekend. The result is generally cloudy skies with periods of scattered showers. The weekend at least looks nicer, with clearing skies, however temperatures will continue to run below normal ... and MUCH below where we have been for the last several weeks. Highs will generally be in the 50's, with lows in the 30's.

Posted by: splillo | April 17, 2012

Drought

A quick verification of my forecasts from the last post ...

Multiple severe weather episodes occurred in the Plains last week culminating in a major tornado outbreak on Saturday. And ridging returned to the East, featuring sunny skies and near record warmth.

Whether you've noticed or not, New England has fallen into a moderate to severe drought so far this spring. This is a combination of a number of factors.

To begin with, the well below normal snowfall this winter has left us with very little snowmelt in the spring. Normally this time of the year, the rivers are bursting with fresh cold water pouring from the mountains. Spring is the season greatest prone to significant flooding for this reason alone. Not this year.

In addition, we have been under a monster ridge of high pressure this spring. The ridge pushes storms to our north, keeping precipitation out of the region (see the image above, courtesy of the HPRCC). It also features incredible heat, which I'm sure you have noticed. March 2012 was not only the warmest on record for the United States, but also shattered 25 individual state records across the country. April has just continued where March left off. This past weekend featured near record breaking high temperatures across the Northeast. All the while, the sunny warm conditions evaporate water from the ground, and the rivers run lower and lower.

There's a saying, "drought begets drought" which means that once a drought begins, it tends to support itself and it's tough to break. Dry ground heats up faster, expanding the high pressure ridging, generally preventing precipitation, and we just get drier. It's a feedback loop that's tough to reverse. This is why you hear of places in the U.S. and around the world with multi-year droughts. These are extreme cases, and I don't think we're not heading in that direction, but at some point we need to get some appreciable rainfall to break the downward spiral. An event of this nature may be in the forecast.

Model guidance has a number of disturbances riding east from the Pacific which could bring some showers to the area by the weekend ...by no means a drought-buster. However by early next week, the forecast models have a significant trough developing in the East, pumping moisture from the Gulf of Mexico into New England. This is a long range forecast, so some changes are likely, but the signal remains for a notable storm next week.

Posted by: splillo | April 9, 2012

Stormy week ahead for the Plains

We are facing another pattern shift coming up this week. Two significant changes can be expected:

1) A break down of the negative NAO block in the Davis Straight (see my last post). This block is anchoring an upper level low over the Northeast which is responsible for the cool and windy weather in the last few days, and will be the cause of some showery weather this week.

2) A significant trough dropping into the western U.S. by the middle of the week. Fairly strong ridging over western North America right now will be breaking down and shifting northeast into the Midwest as multiple disturbances move onshore from the eastern Pacific.

The most immediate effect from these changes will appear in the Plains this week. As these storm systems move across the Rockies, they will set off multiple rounds of severe weather. Below is the forecast surface chart from the Hydrometeorological Prediction Center for Thursday. Notice the frontal boundary across Oklahoma and the dryline in western Texas. These act as triggers for severe thunderstorm development. There's also a tight pressure gradient up through the Plains states supporting strong south/southeasterly flow carrying warm moist air from the Gulf of Mexico.

The next change that will occur with the pattern shift will be warmer temperatures returning to the Northeast. The upper low over us now will be allowed to kick out, and the ridging in the western U.S.  will shift into the Midwest and eventually spill into the East by the weekend. Now, this warmth will not even come close to that of the March heat wave, but it will feel a little more spring-like. Disturbances riding up and over the ridge could still deliver rounds of clouds and showers.

Outside of the contiguous U.S., Anchorage, Alaska officially has their snowiest winter on record after a storm dropped 3.5" yesterday. The image to the right is courtesy of the NWS in Anchorage. Winter is far from over there also. Troughs digging in from the North Pacific will continue to threaten Alaska with rain, wind and snow next week, especially as we see this pattern shift.

Posted by: splillo | March 31, 2012

What we wanted all winter

As we move ahead into April, I want to take a moment to reflect on the historic month of March 2012.

Here's an animation of locations which exceeded record temperatures (courtesy of coolwx.com). Record highs were broken on many days during the month of March. The main "heat wave" started on March 14th and lasted though the 24th. I put heat wave in quotes here because the technical definition is at least three consecutive days exceeding 90 degrees. This was not achieved. But for this warm spell that was likely a 500-year event, I think the term heat wave is justified.

The records "pulse" up during the afternoon since that's when the highest temperatures are reached, however notice in this animation that some locations exceeded record high temperatures even during the night. During the daytime, incredibly large regions of the United States reached record levels.

Following this historic event, you may have noticed a bit of a change in the weather. We're back to winter. Or at least something closer to it. Just 48 hours after basking in 80 degree warmth, we were rudely thrown back into the 40's. In fact, these temperatures are normal for this time of the year, but in comparison it felt like a new ice age.

Moving ahead into April, we can not rule out more winter. Tomorrow is the 15th anniversary of the April Fool's Day Blizzard of 1997, which dropped over 20 inches of snow across much of southern New England. So significant snowfall in April is certainly not unprecedented. Currently, we are in fact heading toward a pattern that is conducive to more wintry conditions. This pattern is what we looked for all winter long and never got ... a negative NAO.

NAO stands for North Atlantic Oscillation. The negative phase is characterized by lower than normal pressure over the Atlantic ocean, and higher than normal pressure over Greenland (this is also referred to as the Greenland block). The low over the Atlantic forces the jet stream further south across the eastern United States, allowing colder air to pour into New England. This past winter the NAO was in the positive phase, yielding quite opposite conditions (warm and snowless). We wanted that negative NAO instead!

At least for the first half of April, a negative NAO should result in generally near or below normal temperatures and even opportunities for snow.

Posted by: splillo | March 19, 2012

Like getting a blizzard in October

My forecasts from the last two posts have certainly verified. The heat is on!

Yesterday, Concord, NH started off at 29 and rose an incredible 52 degrees during the day to reach a high temperature of 81. This absolutely demolished the old record of 64. The average is 44. The high of 81 is the normal for June 21st!! Concord was just one of many cities that broke records yesterday.

This is just the beginning.

Heat has overtaken much of the nation, and the core of the warmth is still yet to come. Look at some of the departures from yesterday (courtesy of weather.com):

Temperatures only continue to climb for the next three days. For central New England this means highs consistently reaching the 70's and 80's through Thursday. For coastal areas, we always need to keep an eye on the seabreeze. Especially this time of the year, the cold marine layer can penetrate well inland and result in low level clouds, stiff easterly winds, and temperatures plummeting from the 70's to the 50's.

What is especially remarkable today is that in Boston, the seabreeze kicked down the door around noon, and yet they still were able to reach another record high. This is a testament to the extraordinary magnitude of this warm air mass.

Thursday is expected to be the warmest day of this period. Temperatures start out in the 50's for most (already well above the normal highs!) and will soar into the 80's across the entire region. A few areas could even take a shot at 90! In March!! To give a little more perspective, these temps are around 40 degrees above normal for this time of the year, and would be above normal even in the middle of the summer.

So what does it take to reach 90 in March?

  • Lack of foliage on the trees promotes warmer temperatures, because less heat is taken away for evapotransporation.
  • No snow cover around the region at all, which is unusual for March in New England. So the albedo is lower, and less heat is taken away for evaporation.
  • Rainfall has been running below normal, so the ground is relatively dry, so again less heat is taken away for evaporation.
  • Increasing westerly winds help "mix" the atmosphere which results in low level compression warming ... just another factor that bumps the mercury up a notch.
  • Finally, the airmass itself! The best measure of the airmass is the temperature aloft (around a mile above the surface). This air then mixes down to the surface, warming through compression. In this case, the temperatures aloft are in the 50's, which is extremely rare for March.
All of these factors will come together to support one of the warmest days ever recorded in the month of March on this Thursday. The statistical rarity of an event like this is perhaps comparable to getting a 20" snowstorm in October...
... as if that could ever happen ;-)
Posted by: splillo | March 12, 2012

The heat ... and thunderstorms?

The normal high temperature for March 12 in Concord, NH is 44F.

Already at 10am this morning, the temperature in Concord is up to 46 degrees. Clear skies will maximize solar heating today, just like yesterday, except that we're already starting a full 10 degrees warmer than yesterday! The high temperature reached 58 yesterday and today we will easily soar into the 60's. Further south, across Connecticut and portions of Massachusetts, highs could even hit 70!

The record high for today in Concord is 63F ... this should easily be broken. The record for Hartford, CT and Boston, MA are both 69F ... these will likely also be broken.

The calm sunny weather will take a hiatus tomorrow. A storm system currently spinning over Wisconsin will track eastward tonight. Southerly flow and warm advection will increase ahead of the storm, which will provide the forcing for some showers tonight into early Tuesday morning. We should dry out by the late morning, but the big question is how much clearing can occur. The southerly flow will certainly get us up to 60F, but if we can get some sun even for just a few hours, temperatures could soar far into the 60's and 70's. A cold front will approach in the afternoon (see the forecast map on the right for Tuesday evening). Combined with the warmth and the moisture from the morning rain, we could in fact have the recipe for a few thunderstorms! These conditions are much common to May than March.

Following the passage of the front, a "cooler" airmass will move into the region for the rest of the week. By "cooler" we're talking temperatures in the 50's instead of 60's ... still 10 degrees above normal!!

By the early next week, forecast models put is right back in a massive ridge of high pressure. In fact, this round of warmth could be even more significant than today! The image below is a model forecast projection for Tuesday 3/20.

For our purposes here, the red is much above normal temperatures. In addition, notice that black line below the red line that extends up through Pennsylvania into New York. Notice where that line is to the west and east ... Mexico and the Caribbean. This is a rough indication of the extent of the warmth we're talking about! Should this forecast verify, we would see widespread 70's and some areas even reaching 80 degrees!!!

Posted by: splillo | March 5, 2012

The heat is on

The weather is not calming down anytime soon. Following two major storm systems that affected the nation in the last week, the Northeast is covered in a fresh layer of snow and ice, and the Ohio Valley down through the Southeast is cleaning up after a major tornado outbreak. I won't go into many details; The news coverage has been excellent for this outbreak. It would appear that 2012 is taking over where 2011 left off. Hopefully this is not a sign of things to come this spring. Below I have included a map of all the severe reports from March 2nd (Source).

Here's an aerial shot of damage in Marysville, IN (Source).

The first half of this week should be calm and dry for this ravaged area, helping with the clean-up efforts. However, by Thursday, another significant system will be swinging through. There shouldn't be any major severe weather associated with this storm though; only some showers and thunderstorms with the frontal passage. Ahead of this storm, temperatures will sky-rocket. A large area of high pressure over the western Atlantic will pump warm temperatures northward. Much of the Northeast will reach into the 50's and 60's on Thursday.

Cooler temperatures will drop into the region behind the front on Saturday. However, this will be short-lived, and by Sunday we'll be back up around 50. The truth is that we're entering a very warm pattern next week. A trough over the Gulf of Alaska will translate to a large ridge downstream east of the Rockies. The image on the right shows a day-10 500mb height anomaly ensemble-mean forecast valid 00z March 15 ... wait, don't stop reading! For our purposes, red = warm. There's lots of red over eastern North America.

The jet stream will be displaced very far north into Canada, flooding the region with warm air. At first glance, it doesn't look like we'll be breaking any records, but this should be a prolonged period of temperatures consistently in the 60's ... closer to that of late April / early May standards!!

Just to cover all my bases, this doesn't preclude any stormy rainy days. In a pattern like this with the jet stream so far north, cutoff lows become more common. Picture a trough ejecting eastward from the Gulf of Alaska into the western U.S. The trough wants to dig southward, but the jet stream resists. What then happens is that the storm digs outside of the jet. Normally disturbances follow the jet stream from west to east across the nation, giving us that general rhythm of storms about once every three days. When a storm cuts off from the jet, there's no longer any force to move it forward. It's like a car pulling into the breakdown lane on a highway. These cutoff lows can linger for many days.

In this pattern, it's possible to get a cutoff situated in the Ohio Valley that would pump Atlantic moisture into New England, producing days of heavy rain. Just another potential to consider in this developing regime. The weather is never simple.

Posted by: splillo | February 27, 2012

Long duration snowstorm

Flakes start flying on a Sunday afternoon; harmless, a gentle winter scene. The storm ramps up overnight, and it rages through Monday with heavy snow piling up against doors and windows. The snow might lighten up once, and everyone thinks the storm is over, only for blizzard conditions to return in full force. It doesn't stop Monday night either, and on Tuesday everyone wakes up to see big fat flakes still tumbling to the ground. There's three feet of snow on the ground, and the storm doesn't wrap up until later that afternoon.

That's my dream storm.

Well we may be looking at a long duration storm threat by the middle of this week. It's not exactly on the magnitude of that in my dream, but a few forecast models suggest we might be looking at continuous snow for a good 24 hours from Wednesday evening through the day Thursday.

A major trough digging into the western U.S. tomorrow will track across the Rockies and support the development of a strong low pressure system in the Central Plains on Tuesday. A large precipitation shield will develop with this storm, stretching from the Ohio Valley back into the Northern Plains by Wednesday morning.

There are two distinct components to this storm.

The first is what is often termed a "southwest flow event" which, as the name suggests, occurs with southwest winds aloft. The forcing for precipitation in this situation is from warm moist (low density) air flowing in from the southwest and forced to rise over cold (high density) air. The warm moist air condenses as it rises, forming clouds and precipitation within the cold airmass.

The southwest flow ahead of the trough will produce this effect, supporting the rain and snow moving from the Ohio Valley into the Northeast ... well ahead of the low pressure system. However, most of the precipitation from this round will probably miss us to the south during the day Wednesday.

The second component of the storm is with the upper level low. The cyclonic flow around the upper low (remember that term "vorticity"?) supports bands of snow (what is often referred to as the "comma head" in a storm). Depending on the organization of the system, these bands can consist of light intermittent snow showers or heavy snow that produces significant accumulations. There are two areas where banding is favored ... in the northeast quadrant and the northwest quadrant of the storm system. So in general, you want the upper low (or "vorticity maximum") to track just south of you for the heaviest snowfall. So the next time you're hoping for a snowday, you can say "boy, I sure hope the vorticity maximum tracks south of us!"

The upper low will roll into the Great Lakes Wednesday night introducing these bands of snow into New England Wednesday night. As the low sticks around on Thursday over New England, so will the bands of snow. The big question is how organized this system will be ... will we just get light snow showers all day amounting to a few inches, or will we get bands of heavy snow that produce as much as a foot by Thursday afternoon? Too early to tell right now, but I can tell you the weather will certainly be maintaining a wintry appeal in New England this week.

Posted by: splillo | February 18, 2012

Storminess and Futility records

Following up from the last few posts, the contiguous US (CONUS) is being affected by a stream of energy pouring onshore in the west and then amplifying over the center of the nation. The storm in formation in the south right now is the first of what will likely be a train of storms pounding the country over the next few weeks.

Why is this so? The storminess stems from something called the Madden Julian Oscillation (MJO) which is essentially a wave of enhanced vertical motion and moisture generating thunderstorm growth near the equator. The MJO has a period of about 45 days and can be tracked around the world, but is normally focused in the Indian Ocean and West Pacific. In the current case, it has propagated into the eastern Pacific and is now energizing the jet stream from the Pacific into the United States.

That's on the large scale. On a smaller scale, we're still staying dry in the Northeast this weekend as the storm is suppressed south of us. This is an impressive system though. A plume of moisture can be seen on water vapor imagery (below) from the Gulf of Mexico across the southern states. Severe weather and tornadoes have been reported from Louisiana to Georgia.

Also visible on this satellite image are the individual shortwaves at work. Notice the circulations in southern Manitoba and Nebraska. Both of these waves will be dropping into the southern system tonight and tomorrow, amplifying the storm. Any other winter, this could be a major snow storm for the east coast, but it just can't snow this year. The problem is another shortwave over the Great Lakes that will be keeping the storm on an east-northeast heading off the Mid Atlantic coast. Areas from Kentucky to Virginia could see a 5" to 10" snowfall tomorrow afternoon ... more snow than Boston has seen all winter.

Boston currently can account for 7.8" of snow this winter, a far cry from the 71.2" they boasted this time last year. The normal year-to-date is 29.8". Of particular interest now is what may be referred to as the futility record of snowfall. Boston's least snowy season on record was 1936-7 with 9.0" that winter.

So in the interest of record breaking, why not just head right on into spring and officially claim this winter to be the king of dead ratters! The problem is New England has a ways to go before declaring an end to the winter that never began. Climatologically speaking, Boston still has 12" to go. Normal snowfall in March is 8.1". And all that's needed is 1.3" to keep this winter out of the record books. The rest of the major sites in New England, like Hartford, Worcester, and Concord, are not as far down in the snowless ditch as Boston, owing to the historic October storm.

Any snow in sight for New England? ...Seems to be a theme of my posts thus far. As long as there are storms, there will be the possibility of snow here. The general pattern being advertised by the forecast models is a broad trough over the eastern two thirds of the nation, which can support fast moving light to moderate snow events here. We'll have to keep an eye on the train of storms over the next few weeks to see if anything can produce.

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