Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Baltimore Maryland/Washington District of Columbia
1017 am EDT Wednesday may 27 2015
high pressure will remain offshore this week. A southerly flow
will bring warm humid conditions to the middle Atlantic through
Saturday. Showers and thunderstorms will be possible each
afternoon and evening. A weak cold front will approach the region
Thursday but wash out before it makes it through. A stronger cold
front will arrive Sunday.
Near term /through tonight/...
temperatures are rising into the upper 70s in a humid airmass this
morning. Showers will continue to move S-SW to north-NE slowly moving
eastward across The Highlands this morning. Breaks in the clouds
will continue east of the Blue Ridge and instability is expected
to increase. As the wave of showers progresses
eastward...thunderstorms are expected to form late this morning
into the afternoon as it taps into unstable air. The wind field
is greater near the Mason Dixon line so strong to severe
thunderstorms are expected closer to PA. Hi res guidance is
depicting thunderstorms to form along the wave of showers that is
slowly heading into The Highlands into the afternoon.
Thunderstorms should move to the east reaching the I-95 corridor
after 2pm. A slight risk of severe weather is expected for areas
east of the Blue Ridge today with the main threat to be damaging
winds. Heavy rain is also possible with thunderstorms.
Incrsg coverage in the convection by midday. The question the past
few days had been how unstable the atmo would become this afternoon...and
as a result what would be the potential for stronger to severe thunderstorms.
While shear has strengthen slightly on the latest model
runs...generally around 20-25 kts...instability remains the bigger
player in the development of the stronger storms for today. Axis of
highest instability aligns along the Lee surface trough this afternoon...with
the thinking that a thinner layer of clouds will set up over the
eastern areas...leading to better diurnal heating. The NAM...with
the typical higher dew points...has the much higher SBCAPE values...above
2000 j/kg for instance at dca. But even the GFS is showing around
1000 j/kg...so even being conservative there is a good amount
instability. Generally thinking that with the near unidirectional
wind flow...will see development of a line of storms over central Maryland
midday under the right entrance region. As the line pushes
east...expecting it to hit the Lee trough aligned just east of the
Blue Ridge...and though lacking on the shear...the instability will
be enough to trigger stronger to possibly embedded severe storms. With
the lack of shear...expecting gusty to damaging winds to be the main
threat with these storms.
By this evening...with loss of surface boundary and instability...
expecting the precipitation to taper off...with areas west of the Blue
Ridge becoming dry during the first half of the night...and everywhere
else by the second half of the night. Another mild night can be
expected with temperatures in the 60s to near 70 in the main metropolitan areas.
Short term /Thursday through Thursday night/...
another day with showers and thunderstorms as surface low currently over the
Great Lakes lifts to the north and east...dragging its associated
cold front into the region by Thursday morning. However...the surface
high and upper level ridge will work to stall the progression of the
front...with it becoming nearly stationary for most of Thursday across
the northern half of the County Warning Area...eventually weakening by Thursday
night. This surface boundary will be the trigger mechanism for
activity during the day as the atmo becomes decently unstable yet
again with temperatures pushing well into the 80s and dew points persisting
in the 60s. While the instability wont be near The Levels as
today...expecting a but more shear over the area to aid in thunderstorm
development. Overall severe parameters on the weaker side...and with
a lacking of upper level support as the better forcing remains north
of the area...not anticipating severe or even stronger thunderstorm
As the frontal boundary dissipates Thursday night...along with any
daytime resulting instability...precipitation is expected to taper off from
north to south. Have maintained at least slight chance probability of precipitation through the
night everywhere...but these could very well be overdone...with the
precipitation tapering off sooner. Slightly cooler temperatures Thursday night behind
the front...still in the 60s though.
Long term /Friday through Tuesday/...
surface troffing will be south of the area by Friday morning. Any thermal
discontinuity would have washed out by then...and middle level heights start to
rise once again. The Bermuda high type synoptic pttn will be returning in
relatively weak surface flow. Will once again have the risk for diurnally driven
shra/tstms...but support for numerous or mature storms seems to be
lacking. Have probability of precipitation no higher than chance...initiating in the higher terrain by
middle afternoon and migrating toward the coast. However...the loss of sunshine
should spell their demise...and probability of precipitation along the western shore of the Bay will
end up lower than inland areas. Maxt will be a cpl degf lower than Thursday. Will
be keeping min-T in the middle-upper 60s due to debris clouds. That could
potentially end up being a pinch too warm based on projected dewpoints.
A stronger cold front will slowly March toward the area this weekend. Will be in
the SW flow ahead of fnt Sat...W/ greater instability being pulled northward
ahead of boundary. The instability axis will remain west of the mountains...which is
where the hiest probability of precipitation will be placed /aided by orographic lift/. In
addition...Sat has the best chance at reaching 90f. For now will be
holding maxt just shy of that.
The fnt will be dragged across area on Sunday...W/ northern stream lopres
heading toward the Canadian Maritimes. That will make the orientation of the
fnt more perpendicular to upper flow. Exact timing still a bit up in
the air...but rafl rates will be a concern since precipitable water will be near 2
inches. Have likely probability of precipitation and slightly lower temperatures.
In a pttn such as this...its always qstnbl how far S fnt will make it
before stalling/washing out. Have cooler air in forecast for Mon-Tue...but
am not sure whether focus for storms will completely push through.
Therefore...have chance probability of precipitation through extended forecast.
Aviation /14z Wednesday through Sunday/...
generally VFR conditions for the taf period. Possible MVFR ceilings
around 12z this morning with ceilings around 2kft currently noted just
to the south and west. Any MVFR ceilings expected to scatter out by
14/15z...with scattered cumulus and broken-overcast middle level deck then expected.
Moderate confidence on the precipitation timing...could see +/- 1-2 hours. While
currently have VFR conditions with the thunderstorms in the vicinity mention...cannot rule
out MVFR or lower conditions with any stronger thunderstorm...especially
District of Columbia/Baltimore metropolitan taf sites. Some storms may also produce
gusty-strong winds and heavy rains briefly.
Precipitation tapers off this evening...with dry weather then tonight. Another
round of precipitation will be possible on Thursday...bringing yet another chance for
Winds generally southerly today 8-10 kts...becoming less than 5 kts tonight.
Winds shift to the northwest tomorrow...though still remaining less than 5
Chance of showers/storms exist each day in the outlook...especially
during the afternoon/evening hours. Best chance for storms will be on Sunday. Local
flgt restrictions to IFR or lower exist in vicinity of storms.
Small Craft Advisory remains in effect today for all zones except the upper tidal
Potomac. Have added the upper zone of the Chesapeake Bay for this
afternoon to account for southerly channeling. For the latter half of
tonight...only keeping the southern Chesapeake Bay zone...with
sub- Small Craft Advisory conditions then expected on all waters by Thursday
morning...lasting through Thursday night.
Thunderstorms expected this afternoon could bring period of gusty to strong
Winds will be on the light side Ely Friday...but S/southwesterly flow will reassert
itself by aftn-evng...continuing through Sat. Channeling will come close to
Small Craft Advisory by Friday evening...but think that Sat afternoon-evening has a better Small Craft Advisory risk.
Cold front will sag across area Sunday...making for numerous thunderstorms and rain.
District of Columbia...none.
Marine...Small Craft Advisory until midnight EDT tonight for anz531>533-
Small Craft Advisory until 6 am EDT Thursday for anz534-543.
Small Craft Advisory until 8 PM EDT this evening for anz530.