Severe Potential Update

Just a very quick update as the SPC has updated their graphics as well.  There is now a moderate risk area outlined that is just to the south and west of the immediate Harrisburg area.  Take a looksie...

Very warm temps have made their way up to central MD and the DC region.  The higher dew point temps have been held at bay due to the persistent onshore low level easterly wind from the cooler Atlantic Ocean.  That is quite a surface low pressure over northern Ohio!  Note the red isotherms pushing northward just to the west of the Chesapeake Bay!  Our dew points remain in the upper 50s.

As a result of the warmer and much more humid air, the CAPE values are also increasing.  Here's the latest map. 1,500 J/Kg is the usual threshold for strong thunderstorm development.  Values much higher indicate a near certainty!  2,500 J/Kg would be considered extreme...as in south central VA at this time.

And here is how the CAPE (Convective Available Potential Energy) has changed since 10 am this morning...

We are still looking for heavy rain to break out later this afternoon into the evening hours.  Here is the rainfall total as per the NA from the wx system that will pass through Friday evening...Again, very difficult to model exact locations as to where the banding thunderstorms materialize...

Enjoy your week's end!

Smitty

AA:  Still monitoring the severe wx threat later today...

A Very Tricky Severe Wx Forecast

To be sure, heavy rain looks like a near certainty during the next 18-24 hours; however, the severe wx parameters of hail and tornadic storms is a much tougher call...as is so often the case.  But especially so today since the low levels are quite stable in our immediate Harrisburg area as the dew points are in the mid 50s and the temps are in the low 60s at present (9:00 am).  This keeps our atmosphere stable and inhibits convection or the development of the potentially severe weather.  As does our current ENE wind.  The longer this coolness stays, the more difficult to create severe instability!    Here is a current map of surface dew points.  Very humid air (energy) is located as close as southern MD. 

And as the humidity gets drawn northward into the surface low over NE Ohio, this should allow for the warmer and more humid air will quickly move into southern PA.  Take a look at how the CAPE has increased over the last 3 hours.  CAPE is simply a measure the potential for convection...

Note how the CAPE and the increased instability is creeping northward into southern PA.  There will also be a very strong jet in the low-mid levels of the atmosphere modeled for later today.  This will increase the likelihood for enhanced upward motion and ultimately severe wx!  The gold arrow points to our approximate location.  This is modeled for ~ 8 pm this evening.  Although the modelers are outstanding in their trade, it is very difficult to pinpoint the exact locations of these potent pockets of fast winds!

Wind shear is also a concern.  Shear has many meanings, but the change of direction with height is critical in determining whether severe weather will occur.  Here is a plot showing the wind shear over KMDT later this evening...this shows the winds from the surface to the 35,000' level.  There is quite a bit of wind shear in the first 10,000' of the atmosphere.  This is conducive for creating storms that rotate or possibly produce tornadoes.

The Severe Storms Center still has us in the "Slight" category for overall severe wx but here is the probability map for tornadoes to occur within a point of 25 miles within the shaded area...

I believe straight line wind will be our largest concern with the approaching front later this evening.  I also believe that 1.50" of rainfall is a good bet!  As does HPC with their QPF through tomorrow morning at 8 am...Also note more tropical troubles in south FL!

One thing for sure is that the next week to 10 days will be cooler than it has been recently.  Here is the 7 day running mean ending next Sunday...quite cool for the Mid-Atlantic and NE quarter of the US.

With all of that, get through the storms of Friday afternoon and overnight, enjoy your weekend and the upcoming cool 1st week of summer!

Smitty

AA:  Potential for some nasty storms Friday evening into Saturday early morning...then a cooler than normal week the 1st week of June!

Severe Wx Potential For Friday PM

This is simply a quick "heads-up" as to the very real potential of severe weather affecting us in the Harrisburg area for Friday afternoon and into very early Saturday morning.  A vigorous surface storm with its attendant fronts will materialize thanks to strong upper air support especially at the 500 mb level as shown below...

The closed low in the mid levels will push a strong jet over central PA late Friday afternoon further enhancing the upward motion that is necessary for severe wx events.  In addition, a triple point (occluded, warm, cold fronts meet) will saunter across PA late Friday afternoon and should be the focus of the greatest surface forcing that will produce heavy precip...Take a looksie.....

The precip that affects us here in PA will come in 2 stages I do believe...the first will be overrunning from the warm front advection and the 2nd bout will be convective as the cold front and triple point low passes over the state.  Take a look at the precip potential for Friday into Saturday...I'd say 1-2" is a very real likelihood by Saturday noon for KMDT!

And the Storms Prediction Center seems fairly confident about the possibility of severe wx.  That is a rather large area for "slight" from 2 days out for the SPC.

And note that we are in the 30% region for severe wx to occur within 25 miles of any point within the shaded region.  The majority of the "red" area is associated with the passage of the cold front Friday evening into early Saturday.

So just a quick heads up as to the severe wx potential....and have a good end of your work week!

Smitty

AA:  Potential for heavy rain and severe weather for Friday pm into early Saturday.

Much Cooler Weekend On Tap

The recent heat will be knocked back substantially over the next couple of days thanks to upper level troughs that continue to work their way in from O Canada.  Last evening's storms were a warning shot that the cooler air is now most decidedly on the move and advecting SE towards PA.  Here is the daily mean temp anomaly for Saturday.  For us in eastern PA, we are looking at ~8ºF below norm day which would keep our high temps right about 70ºF!

Part of that will be the residual cloud cover which will remain from the overnight rains of Friday evening into early Saturday...

But by Sunday, we are looking at a very nice late spring day with max temps in the upper 70s with low humidity...

The only other thing I've got to say is I sure hope I'm sitting at the York Revolution Stadium (SBS) Thursday evening with the CD Ram baseball team vying for their first ever District 3 baseball title...

Have a good mid-week....

Smitty

AA:  Much cooler weekend.  Still a shot of a stray shower today.  Beautiful tomorrow!  Clouding up for some late pm Friday rain...clearing and much cooler this weekend.