Wednesday, December 18, 2013

Early look at weekend storms that follow unseasonably warm weather

First things first: NO, I don't know what a TORCON is or how it's calculated.  4?  8?  2?  I have no idea what's better or worse or what I should do if my area is assigned a certain number.  There's been no peer-reviewed research that explains how it is derived and there certainly is nothing that says, "if you are under a 5, do this."  I pay no attention to it, just like I have no idea what the name of snowflake was that fell on New York City yesterday.  Moving on...

So how 'bout those 70-degree temps in our forecast?!  If it were October, I'd be enjoying it much more.  But since it's mid-December, it must mean something on the opposite end of the spectrum is not too far behind!  An even bigger clue to that "something" resides in our forecast dewpoints, or available moisture in the low levels.  A number above 60 (pretty much anytime) indicates enough moisture to produce thunderstorms.  This time of year, 60 is abnormal (in fact, as I type, the dewpoint is 34, which means our atmosphere is going to moisten up in a hurry in the next 48 hours).

That moisture, which will setup our weekend weather system, will ride in on air coming straight out of the Gulf of Mexico on steady, becoming gusty, south wind starting tonight.  Basically, put low 70s temperatures on top of moisture-laden air, throw in a strong cold front and dynamic jet stream wind at multiple layers of the atmosphere, then mix in a low pressure system just to our west, and you have the ingredients for possible off-season severe thunderstorms.

So when's this all going down? Glad you asked.  We'll have one more nice day (tomorrow) and one more not-so-bad day (Friday) before things get interesting.  A few showers will likely break out by early Friday morning and part of the day Friday will be marked by scattered showers and maybe a rumble of thunder, though it will be far from a washout for you last minute shoppers.  We expect highs in the lower 70s with gusty south wind.  By Friday night, the cold front will bisect Arkansas from northeast to southwest and stall out as southwesterly upper level wind blows parallel to the front, effectively eliminating its "push" to the southeast. Rain and some thunder will be likely Friday night, especially along and north of I-40 it appears, as temperatures remain in the 60s all night.

By Saturday, low pressure will form along the front over northeast TX and travel along the front through Arkansas. This is where models differ, not so much on the details of the low as the amount of precipitation that forms ahead of it in the warm sector - or over east AR, west TN, and north MS - during the day Saturday.  This precip would be in the form of thunderstorms and, given the strong winds and shear in the atmosphere, some storms could be strong to severe.  The two most commonly used models in the mid-range (the American GFS and European ECMWF) have been pretty consistent in their own rights, but offer different solutions.  The ECMWF keeps the heaviest precip north of the metro Saturday, while the GFS drops heavy rain on the metro during the day Saturday.

GFS model (left panels) and European models (right panels) forecast precipitation for Saturday morning (top panels) and Saturday afternoon (bottom panels).  Note the GFS forecasts more precip Sat afternoon (bottom left) than the European (bottom right), which keeps heavy rain farther north.  Click for larger images.

Both models bring rain and thunderstorms through with a pre-frontal trough on Saturday night.  Thus, I feel Saturday during daylight hours is the biggest question mark at this point.  Late Friday night/early Saturday should be wet and Saturday night will definitely be wet.  All rain looks to exit quickly early Sunday morning as the trough moves east.

GFS model (left panels) and European models (right panels) forecast precipitation for Saturday evening (top panels) and after midnight Saturday night (bottom panels).  Note the GFS forecasts much more precip Sat evening (top left) than the European (top right), while the models are more similar after midnight. This leads to higher precip totals overall from the GFS.  Click for larger images.

As for severe weather potential, the Storm Prediction Center (which is the national authority on the topic) places the Mid-South and points southwest, back into central TX, in an elevated risk for severe storms on Saturday, mainly afternoon and nighttime hours.  This risk is equivalent to their "Slight Risks" in the shorter term.  For me, the biggest question concerns timing of precipitation and degree of instability.  In other words, will the precip will be confined to areas northwest of us during the day Saturday when instability would conceivably be at its peak?  Or will we get storms to form in the afternoon that could tap into the instability of daytime heating?  Also, will there be enough unstable air around after dark Saturday night when thunderstorms are more likely to result in severe weather?  At this point, it's too far out to say, but certainly bears watching.

SPC's Day 4 (Saturday) severe weather outlook - area that bears watching.

IF severe weather were to occur, the most likely times will be Saturday afternoon into the early overnight hours and the most likely form of severe weather will be damaging straight line wind and a few tornadoes.  This will be another so-called "high shear-low instability" event, in which sufficient wind energy and turning of the wind exists to support severe weather and a few tornadoes, but instability could be a limiting factor.  One thing we are fairly certain of is that there will be a lot of rain, perhaps up to 2-3", which could cause some flooding.  Keep your storm drains clean so water has somewhere to go.

GFS model (top) and European model (bottom) forecast rain totals through Sunday.  The GFS says about 2".  The European says 1.5-2".  Click for larger image.

Bottom line: "Off-season" severe weather events typically catch people off-guard due to the "it can't happen this time of year" mentality.  There are no guarantees that severe weather will occur, but several of the key ingredients will be in place. Think ahead to where your Saturday plans take you and how you will get severe weather information if it occurs.  We'll have more details as the day get closer.  Follow us on Facebook or Twitter for additional insight in the next few days and download the MWN mobile app, activating StormWatch+ while you're at it (via the Alerts tab in-app).  A "wake-me-up" style audio alert might come in handy if severe weather occurs late Saturday night.

Christmas Day P.S. -- If you're hoping for a white Christmas, it's time to set your sights on something else magical.  Sorry to dampen your Christmas spirit, but the answer is a resounding NO.  In fact, after this weekend system clears out, we'll be back to mostly sunny skies and seasonal temperatures for the first half of next week. Not bitter cold and not snow.

Erik Proseus
MWN Meteorologist

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