Vencore Weather
2Mar/15Mon

1:30 PM | ****Wow, talk about “coming in like a lion”!!****

I_nw_g1_EST_2015030212_076
[12Z Canadian model forecast map for mid-day Thursday (blue=snow].

Discussion

Overview
March began with snow and ice on Sunday, more snow and/or ice is coming for tomorrow afternoon and evening and then, after a temporary warmup on Wednesday, a significant snow event is possible from late Wednesday night into Thursday. Before we get into the details on the very active next few days, let us first throw out some new numbers regarding the amazingly cold February which just ended.

February recap for DC, Philly, NYC
At Reagan National Airport in Washington, DC (DCA), February 2015 was the coldest month at any time of the year since January 1994. DCA saw seven days completely below freezing which is the most in any month since 1979. Dulles Airport in Virginia (IAD) recorded its 2nd coldest February ever and BWI Airport in Maryland saw its coldest February since 1934 (source Washington Post/Capital Weather Gang).

In Pennsylvania, February 2015 was the coldest February ever in State College, Erie, Williamsport and Harrisburg. At Philly International Airport (PHL), this was the 7th coldest February ever recorded going back to 1874 and the coldest month at any time of the year since December 1989 (source phillywx.com).

In New York City, February 2015 turned out to be the third coldest February ever and the coldest since 1934. It was the 2nd coldest February ever recorded in Newark, NJ.

Finally, here is just a sampling of places which set their all-time monthly (cold) temperature records: Buffalo (NY), Islip (NY), Hartford (CT), Bridgeport (CT), Syracuse (NY) and Bangor (ME).

Tuesday afternoon and evening - snow, ice threat
After a very cold night in which temperatures drop into the teens, clouds will thicken up early Tuesday ahead of the next precipitation event. Precipitation is likely to break out during the mid-day or early afternoon hours in DC, mid-to-late afternoon hours in Philly and New York City, and it should be cold enough for that initial precipitation to be of the frozen variety. In DC, the precipitation could start as all snow, but a wintry mix is more likely whereas in Philly and NYC the precipitation is liable to begin as all snow. In all areas, any initial snow will change over to a wintry mix fairly quickly later tomorrow or early tomorrow night and, yes, there can be some snow and ice accumulations by midnight. Temperatures tomorrow night should slowly rise and any mixed precipitation is likely to change to plain rain by early Wednesday.

Wednesday – milder with plain rain
Wednesday promises to be a milder day with periods of rain. Temperatures could even flirt with the 50 degree mark on Wednesday in portions of the I-95 corridor. The more important story; however, will be the approach of another Arctic cold frontal system. This front will pass through the region late Wednesday and this will begin a downward trend in temperatures that could set the stage for significant snow on Thursday.

Late Wednesday night/Thursday – potential significant snowfall
Once the Arctic front reaches the east coast, it will stall out due to the alignment of the upper-level winds which will prevent any significant movement to the southeast. At the same time, copious amounts of moisture will begin heading northeast along the stalled out frontal boundary zone and into the I-95 corridor. At the same time, colder air will be pushing in from the northwest and the result could very well be a changeover rain early in the day to an accumulating snow and significant accumulations are possible up and down the I-95 corridor.

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2Mar/15Mon

7:00 AM | ***Week starts off on the cold side…mid-week brief warm up preceded by a wintry mix and then plain rain…cold air returns later this week possible associated with significant accumulating snow***

6-Day Philly Forecast

Today

Mostly cloudy early, partly sunny during the afternoon, becoming quite breezy, cold, highs in the upper 30’s

Tonight

Partly cloudy, very cold, lows in the mid-to-upper teens

Tuesday

Becoming cloudy, cold, snow or a wintry mix likely late in the afternoon, low-to-mid 30’s

Tuesday Night

Snow or a wintry mix gradually changing to plain rain, temperatures slowly rise

Wednesday

Becoming noticeably milder with rain likely, quite breezy, mid-to-upper 40's

Thursday

Cloudy, breezy, much colder, snow likely and significant accumulations are possible, low 30’s

Friday

Mostly sunny, very cold, near 30 degrees

Saturday

Mostly sunny, not as harsh, but still cold, near 40 degrees

Discussion

Our active weather pattern will continue for the next several days with a potpourri of weather conditions including snow, sleet, freezing rain, plain rain, and the chance for a substantial snow accumulation. The new work week start off on the cold side and the winds will intensify during the mid-day and afternoon hours. After a very cold overnight, clouds will thicken up early Tuesday and snow or a wintry mix is likely to arrive during the latter part of the afternoon. Incoming milder air will gradually change the precipitation to plain rain later tomorrow night as temperatures slowly rise through the 30's. Wednesday promises to be quite mild ahead of the next Arctic frontal system with periods of rain and temperatures reaching the 40's.

That Arctic frontal system will slow down as it reaches the east coast and will set off a chain of events that could bring a significant snowfall to the I-95 corridor. Copious amounts of moisture will ride along the frontal boundary zone Wednesday night and Thursday as colder air pushes into the Mid-Atlantic region. This type of setup could feature a cold rain that changes to snow by early Thursday with significant accumulations possible. Very cold conditions will follow for the end of the work week.

Looking ahead, next weekend looks to be moderately cold and then another shot of Arctic air is possible early next week. Beyond that, there are signs for an important break in the pattern centered around the middle of the month with several days of milder weather conditions. However, it is quite likely that we'll return to a colder-than-normal pattern - perhaps stormy as well - during the latter part of the month and going into April.

Video

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1Mar/15Sun

8:45 AM | ***Accumulating snow and the threat for a significant buildup of ice***

us3comp
[US radar image with plenty of moisture headed to the Mid-Atlantic region; NEXRAD image courtesy University of Wisconsin]

Discussion

Overview
Dense, cold Arctic air sits on top of the Mid-Atlantic region this morning and copious amounts of moisture (radar map) is streaming in from the west – a bad combination that spells big trouble for the I-95 corridor region from DC-to-Philly-to-NYC. Accumulating snow is likely in all areas along I-95 at the onset of today's precipitation event, but that ultimately will not be the biggest problem. Milder air moving northward in the upper part of the atmosphere will generate a changeover from the snow to sleet and then to freezing rain in most areas and there can be a significant buildup of ice on untreated surfaces (e.g., trees, grass, untreated roadways) with slick travel conditions. While the upper atmosphere turns milder, the dense, cold and below-freezing Arctic air at ground level will be reluctant to retreat; thereby, setting the stage for some serious ice buildup later today and early tonight in portions of the area.

Details
Snow is likely to break out by mid-morning in the DC metro region perhaps even mixed with some sleet in southern sections, in the late morning to mid-day hours across Philly, and then during the early afternoon in the NYC metro region. Then, after a period of accumulating snow, there will then be a transition later today from south-to-north of the snow to sleet and then to freezing rain although snow will hang on for much of this event in higher elevation locations north of I-80 (e.g., NE PA). Before the changeover to ice, snow should accumulate from a coating to an inch or two in and around the DC metro region, 1-3 inches in the Philly metro region, and 2-4 inches in the NYC metro region. The higher amounts in those snowfall accumulation estimates will occur to the north in a given metro region and the lesser amounts to the south. A significant ice-buildup (quarter to half an inch) is possible later today into early tonight in the entire I-95 region from DC-to-Philly-to-NYC. Precipitation winds down by or shortly after midnight in most of the I-95 corridor.

“Banding” could enhance snow (or ice) totals in parts of the area
One final word of caution on today’s event…there will be an impressive thermal gradient in the Mid-Atlantic region and strong upward motion in the atmosphere as a powerful upper-level jet streak (purple area on 300 millibar map) plows through the region from west-to-east. As a result, in this type of dynamic atmosphere, “banding” is likely to develop in the precipitation field later today causing varying rates of precipitation and there could be an enhancement of the snow (or ice) in a given area in a short period of time. We’ll monitor the precipitation pattern as the day progresses and make adjustments to accumulation totals if necessary.

300mb_sf
[Sunday AM 300 millibar winds showing powerful jet streak (purple) which is headed right into the Mid-Atlantic region; courtesy NOAA]

Another interesting and complicated threat at mid-week
Another complicated precipitation event is likely to occur in the I-95 corridor from Tuesday into Thursday. Precipitation ahead of another strong Arctic frontal system is likely to arrive late Tuesday and it could start as a wintry mix in the I-95 corridor. However, as milder air pumps in from the south, the precipitation is likely to change to plain rain for Tuesday night and early Wednesday and some of the rain can be heavy. Temperatures could climb into the 50’s in portions of the I-95 corridor during the day Wednesday before colder air returns. The Arctic frontal system may then stall as it reaches the east coast on Wednesday night and there is a chance low pressure develops along the frontal boundary and rides up along it into the I-95 corridor region. If this takes place, it could very well turn cold enough for some snow around here in the late Wednesday night/Thursday time frame. Stay tuned on that one.

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28Feb/15Sat

12:00 PM | **Looks like a real mess on Sunday with a buildup of snow and/or ice depending on location; another complex and interesting precipitation event comes in the Tuesday-Thursday time frame**

usfntsfcwbg
[Mid-day US surface weather map with very strong Arctic high pressure centered right over the Mid-Atlantic; courtesy NOAA]

Discussion

Overview
The weekend begins with very strong Arctic high pressure sitting right on top of the Mid-Atlantic region (surface map above) assuring us plenty of sunshine today along with light winds. Atmospheric pressure readings at mid-day are as high as 30.85 inches (1045 millibars) in parts of the Mid-Atlantic region associated with this very dense and cold Arctic air mass. The combination of this entrenched dense, cold dome of Arctic air and moisture streaming in from the west on Sunday spells big trouble for the DC, Philly and NYC metro regions with accumulating snow or a significant buildup of ice or both real threats as we close out the weekend.

Details
As the high pressure shifts off the east coast on Sunday, a warm advection pattern will set up with light snow likely to break out in the pre-dawn hours across western and central portions of the Mid-Atlantic region. During the mid-morning to late-morning hours, snow is likely to break out in the I-95 corridor and it will likely change to a wintry mix in the DC metro region after a few hours, but not before there is a coating to an inch or two of new snow accumulations. The snow will continue for a longer time period across SE PA with 2-4 inches likely before any changeover and areas to the north of I-80 are probably the luckiest of all as the snow up there could hang on for most or all of this upcoming precipitation event; thereby reducing chances up there for any serious ice buildup. The New York City metro region should receive several hours of snowfall before any changeover takes place with 3-5 inches of accumulation possible in that area. The period of icing following the initial “thumping” of snow can produce a significant buildup of ice later Sunday into Sunday night; especially, in the region from the N and W suburbs of DC to the N and W suburbs of Philly. All precipitation winds down in the pre-dawn hours early Monday and the rest of the day remains cold and becomes quite breezy.

“Banding” possibility could enhance snow (or ice) totals in parts of the area
One final word of caution on tomorrow’s event…there will be impressive upward motion at mid-levels of the atmosphere centered around northeastern Maryland and southeastern Pennsylvania late in the day. The 12Z NAM forecast map (below) for early tomorrow evening shows this area of mid-level upward motion (850 millbar absolute vorticity) centered near the eastern part of the Mason-Dixon Line. Often times with this kind of concentrated upward motion, there can be “banding” that sets up in the precipitation field and this could lead to enhanced snowfall (or ice) amounts in a short period of time over parts of the area…something to watch for later tomorrow.

nam_namer_036_850_vort_ht
[12Z NAM forecast map of 850 millibar absolute vorticity early Sunday night; courtesy NOAA]

Another interesting and complicated threat at mid-week
Another complicated weather pattern is setting up for the I-95 corridor for the time period from later Tuesday into Thursday. Precipitation ahead of yet another strong Arctic cold frontal system is likely to arrive Tuesday afternoon and it could start as a wintry mix in the I-95 corridor. However, as milder air pumps in from the south ahead of the front, the precipitation is very likely to change to plain rain for Tuesday night into early Wednesday and some of the rainfall can be on the heavy side. Temperatures could actually climb into the 50’s in portions of the I-95 corridor during the day Wednesday before colder air returns to the region. The Arctic cold frontal system may then stall out as it reaches the east coast on Wednesday night and there is a chance that low pressure develops along the frontal boundary zone and rides up along it right into the I-95 corridor - and into the incoming colder air. If indeed this takes place, it could very well turn cold enough for accumulating snow around here sometime in the Wednesday night/Thursday time frame. Stay tuned.

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