April Fools' Storm A Day Away

We're a bit less than 24 hours away from the April Fools' Storm's arrival in the region, with a soggy start and for those north & west of the city, a snowy ending as the storm intensifies tomorrow night and early Friday, leading to a transition from rain to snow for higher elevations north of the city.   The potential does exist for significant accumulations in the Poconos, the higher elevations of the Lehigh Valley, and North Jersey -- Winter Storm Watches are out for those areas.  

For the city, the "best" chances for snow will come at the end of the event on Friday morning as the heaviest precipitation is pulling away and colder air pulls in aloft.  We might get a brief mix in the city and the immediate burbs.   Computer guidance is suggesting that snow should accumulate to the north of the city -- after starting as rain -- as the storm intensifies and cold air ingests into the storm's western flank.  However, not everyone on the western flank will see snow.  Elevation helps -- if you're in the Poconos you'll probably get a good bit of snow.  Likewise, those in the upper parts of Bucks and Chester Counties should get some accumulations as well.   However, valleys and the urban core of I-95 will likely stay mostly rain.

Timing-wise, any transition of rain to snow should take place in the late evening or overnight hours on Thursday -- after rain starts up on the front end tomorrow afternoon or early evening.   Snow accumulation could be fast/furious over the higher elevations -- perhaps two inches per hour in the Poconos.   How quickly that snow fires up is dependent on how quickly the storm intensifies.   A slower intensification means less snow overall.   We'll have an accumulation forecast for the storm out tomorrow afternoon.

As far as Philadelphia is concerned, this should be a mainly rain event and should be over early Friday morning...perhaps during the morning commute.  Assuming no unforeseen accumulations of snow overnight tomorrow, the game may be delayed out of the gate but it looks like the Phillies have a decent shot of playing on Friday.   As always, stay tuned.

It May Be Spring, But...

 The calendar and meteorology both say it's "Spring" by definition -- in fact in a meteorological sense it's been Spring for over 20 days.   However, Mother Nature and Old Man Winter don't care to agree with such silly wishes of warmth from most of the masses and wants to send a plethora of precipitation types into the region tomorrow.    Radar is already showing the first fingers of the storm system extending into West Virginia, with the main show of this storm back in Minnesota and Wisconsin.   Moisture that's streaming out east-southeast of the main low is in association with a frontal boundary that's slipping south along the East Coast and will play a huge factor in the storm's track, our temperature, and what ultimately falls from the sky.

This will not be an easy forecast -- the dreaded "nowcast" applies to this storm as temperatures at the surface will arguably make or break your precipitation type and one's forecast.   However, with temperatures poised to be colder to the north that at least some accumulating snow is likely from Montgomery and Bucks County on north, particularly the upper parts of the county.   I'm not so convinced that applies to the city...yet...but I do think it's pretty likely that the city does get some wet snowflakes mixing in.   It all comes down to timing and temperature with this event.   More on that in a minute.

Winter Storm Watches are out for the Lehigh Valley and Berks County on north -- the watches in the Poconos definitely make sense given the profile in temperatures is going to be cold enough for mostly frozen precipitation up there although one can't rule out some rain mixing in from time to time during the day on Wednesday up north.  The Lehigh Valley and Berks, in my opinion, are the "battle zone" where a colder solution such as the one the NAM is painting gives warning snowfall since little or no mixing at the surface occurs.  However, it is the only model painting that as the Euro and GFS both paint a mixed bag event where morning snow mixes and then changes to rain during the day as temperatures warm sufficiently above freezing in most locations.    The surface warming above freezing is the difference between a couple of inches of front-end snow and a six inch snowfall in the Lehigh Valley and even the upper parts of Bucks and Montco.

Farther south, the temperature profile battle as not as huge a deal but the NAM's depiction on temperatures would suggest some snow down into the immediate city on the front end before temperatures moderate enough aloft to turn the city back to rain, whereas the GFS and Euro are more of a mix to rain scenario.

The other concern is how this system wraps up on Wednesday night.  The NAM is suggesting a changeover back to snow in the suburbs, Philly, and even parts of South Jersey but again, it's the only model showing this changeover as the GFS and Euro both wrap precipitation up in the region before temperatures begin to cool off.   It's something to keep an eye on at this point.

Even if it doesn't snow at all in the city, the temperatures won't be all that high and the rain will be rather chilly.  

We'll have a snowfall forecast out this evening for the storm system and we'll update the storm as it moves through during Wednesday.

A Thin Line

A tricky forecast for parts of the area as we move into Wednesday -- one that features the prospect for more snow in parts of the Mid Atlantic and one that features the prospect for more rain as well.   The question is what falls when, how much snow falls where, and temperatures.

This system will have a similar arrival point -- early morning hours -- as today's system and has the potential as it works into a dry airmass of causing some evaporational cooling at the onset despite temperatures at the surface that will be above freezing.  This will result in a mixed bag of rain and snow as close to the city as the northern suburbs, perhaps even into the city itself if the colder modeling from the Euro (below) and NAM are correct.   As we transition into the daylight hours on Wednesday, chilly precipitation will fall off and on during the day...with any mix of snow/rain transitioning over to rain from south to north.

The question is just how far north that transition zone gets -- especially for areas north of I-78 where the GFS depiction warms you sufficiently above freezing to make this an event that's more like Monday and less like a snowstorm.   The NAM is colder than the Euro even, with potentially a foot of snow for the Poconos, a few inches across the Lehigh Valley, and even an inch or two in the northern burbs in a longer duration event that would last through Wednesday night before ending.    The NAM also brings cold air back in during the evening and turns the rain for folks north of the city back into snow.  The GFS cuts those totals down by 60-70%, which just a few inches in the Poconos, an inch or two in the Lehigh Valley, and maybe an inch into Bucks County.

The low's track in this storm will be a bit farther south than in today's storm -- computer guidance generally sends the low along or just south of the Mason-Dixon line.  The reason why the GFS is warmer is because it has a stronger push of warmth aloft and at the surface plus it is the most northern of the tracks, with the Euro south of the GFS and the NAM a bit south of both.  The NAM also "skips" the storm to the coast from Western PA, similar to the Euro but a touch south whereas the GFS doesn't have this skip taking place.

Historically, the NAM does "well" in temperature profiles within 48 hours of an event -- the operative word is within 48 hours.   Based on the midday runs of the NAM that were initialized on data at 8 AM this morning, 48 hours would be Wednesday morning.  After that point, its reliability begins to decline.    I'm not so certain the NAM ends up more right on this event in terms of its final outcome.   However, I do think we'll see some snow at times, perhaps closer than many of us would like.

When it doesn't snow on Wednesday upwards of an inch of rain can't be ruled out across the Philadelphia area, with less to the south.   It's not going to be a "nice" day no matter how you slice it and no matter what ends up falling.

The First of Two Chilled Rains Next Week

After flirting with or hitting 80 degrees yesterday, the wilds of March have to turn the other cheek. Mother Nature can't let us enjoy May-like warmth for too long, can she? Colder air will be on the advance starting today as temperatures drop to more typical levels for March -- daytime highs in the upper 50's. While still nice, it's not quite yesterday nice. The cooler trend will become increasingly established over the next few days as a couple of systems scoot through from west to east, generally across our region, with chances of chilly rain here in the city. Farther north, some snow is certainly possible as these systems work through colder air than what we have here.

Monday brings the first system -- this generally passes through during the morning hours and may start up just before daybreak west and northwest of the city. With some residual cool air lingering around the region and temperatures aloft starting off cool, some snow and/or sleet could fall at the onset north of I-78 before transitioning to rain. South of I-78, some sleet could mix in with a chilly rain on the front end of the system before temperatures warm above freezing aloft and we all see a cool rain.

With temperatures generally in the 30's (blue) and near freezing (pink line) to the north, some frozen precipitation can't be ruled out over the northern parts of the area.  I tend to think the best chances will be Scranton, the Poconos, Northwest Jersey but the timing (morning) and intensity (light) means any accumulating frozen precipitation may be confined only to grassy surfaces and not to roadways up north.

A second system is poised to impact the region on Wednesday night and Thursday.  There is a chance some frozen precipitation could work closer to the city but computer guidance has been trying to keep the frozen precipitation to a minimum in recent runs.   Both of these systems will help to establish a -NAO over the East for the next week or so, with temperatures cooling off quite a bit by the end of next week....where highs could be in the lower and middle 40's as a large trough pivots through the East for the end of March.

Hope you enjoyed the Spring Fling yesterday, folks.  Old Man Winter still has a little more spark in his step yet.

Snow To The North, Not So Much Of Anything Here

Computer modeling for tonight's event is pretty much set on a snowy scenario to our north, with little to nothing around town as we get clipped by a larger surge of moisture that should generally cruise east along I-80 overnight tonight.   Precipitation is working east through the Ohio Valley this morning and should lift a bit more northeast as the day progresses, meaning that most precipitation should fall generally to the city's north overnight.   Timing wise, expect precipitation after 9 PM in Philadelphia.   High resolution radar (below) suggests precipitation starts up around 11 in Philadelphia.

Temperatures tonight will cut it rather close to freezing -- I'm not expecting the city to drop below freezing but there may be enough cold air aloft to allow for some snow or sleet to mix in with any rain in the city proper.  The farther north one goes, the better the moisture support, dynamics, and the better chances one will have for a snowy scenario.  In fact areas north of Scranton could see several inches of snow overnight.   Another concern for the folks in the Lehigh Valley and far northern suburbs is the potential for some icy rain to mix in as temperatures aloft warm before the surface.   The Lehigh Valley, Berks could see an inch of slush with a bit of icy rain on top of that slush.  

Assuming the higher resolution guidance is correct, precipitation won't last long in and around Philadelphia and most of the suburbs, meaning that the precipitation burst that we get tonight will be brief and won't amount to much.  The northern suburbs could end up with some coating type accumulations but widespread major snows aren't likely from round one of the one-two combination that we're getting over the next 48 hours.   The city should not see much, if anything, besides some snow or sleet possibly mixing in.  Even that, at this point, doesn't seem terribly likely but is certainly a possibility.

Due to the potential for more snow farther north, Winter Weather Advisories are out for the Poconos and Northeastern Pennsylvania for tonight and tomorrow morning.

Updating Tomorrow's Sloppy System

Nuisance but could be dicey to get around tomorrow morning.  That best sums up the impact of the next system to come up the pike for Saturday.   It's a nuisance event in the sense that precipitation totals will be modest -- less than a half inch for everyone.  It's also not going to phase together in time to bring an explosive amount of snow to the Poconos.  With all that said, it's still something and given it will be working into a colder airmass we'll probably have some frozen precipitation on the front end that will transition to slop and freezing rain before temperatures rise above 32.

High resolution guidance from the Storm Prediction Center indicates the advance of precipitation into the region by Saturday morning.  It will likely move into Philly after 1 AM and start briefly as snow before the mid levels warm above freezing.  The surface may be slower to warm and as a result some freezing rain can't be ruled out across the Delaware Valley and northern suburbs for a time before temperatures rise above 32 degrees.  That 32 degree point should be reached during Saturday morning, which means slick spots will be out and about.  However, this won't be a repeat of the ice storm event a couple of days ago as precipitation will be modest.  With that said, slick spots will abound during the pre-dawn and morning hours before the sun works the roads through the clouds and temperatures nudge above freezing.   A chilly, off and on rain will linger through the day before ending in the evening hours in the city.

To the north, over the Lehigh Valley, a longer period of icy rain is possible before temperatures reach the freezing point.  You could have some travel issues for a longer period of time but up there you should see improvement in the weather towards midday.   In the Poconos, you will start as snow before transitioning to a period of freezing rain.   Up to our north, you have a better shot of seeing precipitation trend back to snow in the evening as the storm system begins to intensify and upper level energy works east from the Ohio Valley.   This transition back to snow could bring a couple of inches of snow on the back end of the event, especially north of Blue Mountain, with perhaps an inch on the back end in the Lehigh Valley.   Precipitation should end to the north around Midnight if not before.

I've scaled back the snowfall totals a fair bit to reflect a modest event overall, with just coating accumulations possible across Southeast Pennsylvania and South Jersey, with an inch or two from Trenton to Pottstown to Lancaster and north.   Two to four inches of snow are possible in the Poconos and Northern Pennsylvania.

The biggest issue will be the slick travel around during Saturday morning in some spots, especially city and north.  While it won't be a repeat of the ice storm from Wednesday, a little freezing rain can cause more problems than a lot of rain.