Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Baltimore Maryland/Washington District of Columbia
354 am EDT Sat Jul 12 2014
high pressure will shift offshore...allowing southerly return flow
to become established in the middle Atlantic through Monday. A cold
front will cross the area Tuesday. High pressure will build
along the eastern Seaboard for the second half of the week.
Near term /through tonight/...
baroclinic zone was located from the Outer Banks of North Carolina
westward through the southeast states...allowing weak high pressure
to reside across the northeast and Middle-Atlantic States.
Area of clouds was noted over the central Shenandoah Valley and
foothills at 06z...with mostly clear skies elsewhere. Patchy fog was
developing in the northern Virginia Piedmont where skies were mostly
clear. Fog has yet to develop within the central Shenandoah
Valley/foothills where it was locally dense yesterday morning but if
skies can clear by daybreak...then there will be patchy fog here as
For the rest of the day...bands of middle/high clouds visible upstream
over the Ohio Valley will spread east through the day likely
transitioning the sky to partly sunny by afternoon. During peak
heating...despite the cloud cover scattered thunderstorms are
expected to develop near the higher terrain/ridges. It/S not out of
the question that a few storms could push east of the Blue Ridge
later in the afternoon/early evening as suggested by the operational
NAM/lwx WRF-arw...but preferred more of a srefs solution with probability of precipitation
relegated to along and west of the Blue Ridge.
Convection should be diurnally-driven and dissipate with the setting
of the sun. Patchy fog may form again overnight...particularly in
climatologically fog prone areas and also places that receive
Short term /Sunday through Sunday night/...
models have been consistent that moisture and instability will
increase for Sunday. GFS has precipitable water values increasing
close to 2 inches by day/S end...with dewpoints near 70 degrees.
Models also show a decent vorticity maximum affecting the County Warning Area...promoting at
least scattered showers and thunderstorms. While shear still looks
best northwest of the County Warning Area...instability and strength of vorticity maximum may
promote a few strong or severe storms during the afternoon.
Activity from Sunday afternoon should diminish/move away Sunday
evening but will continue with low probability of precipitation across northwest portion of
the County Warning Area as faster advance of a trough/cold front may allow for some
overnight convection within this zone.
Leaned closer to mav for maxima Sunday given southwest flow in the
low levels...mav/met were similar for Sunday night.
Long term /Monday through Friday/...
forecast for the beginning of the weak will be dominated by an incrsgly amplified
500 mb pttn...where shortwave energy will dive across the Great Lakes...pushing a well
defined cold front toward the eastern Seaboard. As can be expeceted west/ any anomolous
solution...variances exist amongst guidance members. Regardless of specifics
there is consensus that County warning forecast area Monday will be west/in wm sector... affected by
a hot/humid/unstbl air mass. A Lee trough axis may provide a ncsry focusing
mechanism...although anothr source of upward vertical velocity would reside across the northwestern County warning forecast area
due to approaching bulk shear and orographic lift. Have opted to place
focus...by way of likely probability of precipitation...there. Expect updrafts to be vigorous
enough to sustain themselves...and have carried likely probability of precipitation into the I-95
crrdr by evening. All ncsry ingredients available for thunderstorms to not only
become severe...but also be organized. Just need to see how it all
Am thinking that the GFS may be a little too quick west/ progression of
cold front. Its timing will ultimately determine the fate for Tuesday. A faster
fnt would curtain severe thunderstorm chances. Too many debris clouds would yield the
same outcome. Either way...the I-95 crrdr stands the best chance at
receiving measureable precipitation...which warrants likely probability of precipitation Tuesday. To the
west...probability of precipitation held back at chance levels. Severe thunderstorm risk for both Monday and Tuesday
will be continue to be carried in the severe weather potential statement.
Maxt Monday simlr to prvs forecast...going a pinch above guidance to acct for
warm 800 mb temperatures and ample insolation. Tuesday maxt nearer climatology due to
expeceted clouds. In fact...west of blurdg a bit below climatology. This forecast period may
have the hiest error potl in the extended forecast. Stayed warm for min-T Monday
night...as dewpoints near 70f will inhibit substantive cooling.
By Tuesday ngt-Wed...cold front will be pushing away from County warning forecast area...permitting Canadian
hipres to build. This high will be the controlling weather influence for the
rest of the weak. Temperatures/dewpoints will be refreshing for July in the middle
Aviation /08z Saturday through Wednesday/...
already have some clouds over cho...with more upstream on the way
for cho and mrb through sunrise. Clouds should help visible from
tanking...thus have allowed for MVFR restrictions /but not IFR/
early this morning. A few thunderstorms may form near the higher
terrain this afternoon...mrb most vulnerable. Some guidance suggests
there could be convection moving east to affect cho/iad later in the
afternoon. Confidence low...and have restricted chances along/west
of the Blue Ridge.
If skies clear out tonight...some fog is possible especially
cho/mrb. Higher coverage of thunderstorms is expected Sunday with
better moisture/instability and a shortwave. Afternoon thunderstorms
will be possible all terminals.
Flgt restrictions likely at times Monday-Tuesday in vicinity of thunderstorms and rain. VFR will prevail
for a majority of the time...but brief periods at or below IFR possible.
Hipres builds Wednesday. VFR.
winds were mainly from southeast to south early this morning...
favoring nearly due south this afternoon into tonight in tandem
with an increase in speeds. Small Craft Advisory is already in effect for much of
the Chesapeake Bay and into the lower tidal Potomac late today
into tonight. There may be a lull during the day Sunday...but it/S
possible winds increase into Small Craft Advisory criteria again Sunday evening or
Intervals of thunderstorms and rain likely Mon-Tue...each posing a gusty wind threat.
Gradient flow may be enough to place waters on the cusp of Small Craft Advisory
anyways. Any addtl wind would necessitate smw/S. The final wave would be
a cold front. At this time guidance not suggesting gusty winds in northwest flow
Post-fropa...but based on temperature contrast...suspect that will change.
There/S a bit of uncertainty on exactly when cold front passage would be. Since thats
at the end of the marine period...will omit at this time. Later forecasts can address
water levels were running close to normal early this morning...but
as southerly flow increases so too will anomalies later today into
tonight. Next couple of high tides should be comfortably below minor
flood criteria...but by the early Sunday morning high tide sensitive
sites such as Annapolis may approach minor flooding thresholds.
District of Columbia...none.
Marine...Small Craft Advisory from 6 PM this evening to 6 am EDT Sunday
Small Craft Advisory from 4 PM this afternoon to 6 am EDT
Sunday for anz532>534-537-543.