Scientific Forecaster Discussion
fxus61 klwx 220801
Area forecast discussion
National Weather Service Baltimore Maryland/Washington District of Columbia
401 am EDT Thu Jun 22 2017
high pressure will remain west of the area today. A cold front
will cross the area late Friday night into Saturday, with
tropical moisture streaming into the mid Atlantic ahead of the
cold front. High pressure will return Sunday and persist through
early next week.
Near term /until 6 PM this evening/...
diffuse frontal zone resides across northern portions of the
area this morning, where isolated showers have been developing.
Hi-res guidance indicates these will be diminishing near or
shortly after sunrise.
Front will lift to the north today. A bit of warm advection
will result in temperatures rising a few degrees over yesterday,
so expect lower 90s to be more common. Mid and high clouds
could be a limiting factor though. Theta-E advection will also
begin this afternoon, with perhaps around 1000 j/kg of SBCAPE,
especially over the mountains. Terrain circulations and a Lee
trough will help initiate showers and thunderstorms this
afternoon. Hi-res models have various solutions on how far east
and south the thunderstorms will make it. Have expanded pops
over previous forecast, but have kept them in the 20-40% range.
Deep layer shear is expected to remain less than 30 kt, so
storms should be mostly pulse variety unless a cluster forms
Short term /6 PM this evening through Saturday night/...
overall setup for the influx of tropical moisture remains
similar from tonight through early Saturday. There will be two
opportunities for a slug of moderate to heavy rainfall (though
likely not long duration), with potential for convection in
between on Friday afternoon.
Lead shortwave/llj/isentropic lift will arrive first tonight.
Have trended pops toward the speedier timing of the NAM/wrfs, as
these types of events often arrive and depart faster than
progged. Low level jet will be transporting pwats of around 2 inches,
which will then remain in place. However, overall duration of
this rain is on the order of 3-4 hours, so even with efficient
rain rates, am thinking flooding will be a low concern with this
round other than ponding. This round of rain should be departed
by mid morning Friday.
Thermodynamic environment remains uncertain heading into Friday
afternoon as clouds will be abundant and atmosphere will
stabilize some after morning rain. However in the moisture rich
environment, some instability is likely to develop. Deep layer
shear will also be increasing as upper trough digs to our west.
Latest guidance suggests the best chance of storms will be
across northern/western portions of the area where convergence
may be a little better along with weak mid level impulses. Will
have to monitor this time period for severe weather as well as
heavy rain rates and flash flooding, though wind fields should
be sufficient to keep storms moving.
Final opportunity for heavy rain will arrive with the remnants
of Cindy and a southward moving cold front late Friday night
into Saturday morning. Once again, the forcing will be fast
moving, so prolonged heavy rain is unlikely. However, rates may
be especially intense given the continued high precipitable water and deep
warm cloud layers. Given ground may be more saturated in some
areas by this point, will have to monitor potential for flash
There is some spread on how quickly the cold front sweeps east
on Saturday. There is at least some chance thunderstorms could
redevelop east of I-95 during the afternoon. However, dry
conditions are expected by Saturday night, along with lower
temperatures and humidity.
Long term /Sunday through Wednesday/...
upper level troughing looks to dominate the region as we head
into next week, centered near the Great Lakes. This will
position a high pressure in a semi-permanent fashion over the
central US. In turn, this will result in a somewhat unusual
northwesterly flow at low levels through at least the first
half of next week. This will result in slowly cooling
temperatures as weak reinforcing shots of cooler air push
southeastward into the region, likely accompanied by little
precip. Bottom line is that it won't feel very much like late
June. The trough and associated cooler air may move away late in
the week as surface high pressure shifts eastward, eventually
assuming a more Bermuda-high like position by next weekend.
Aviation /08z Thursday through Monday/...
little issues through this morning, though some isolated
showers have been near mrb and BWI due to an old frontal zone.
Scattered showers and storms may form near the mountains this
afternoon. Mrb has the highest chance of being affected, though
wouldn't totally rule out something reaching the metros. The
first half of the evening will be VFR.
Main concern the next couple of days will be periods of heavy
rain as tropical moisture flows into the area. One will be late
tonight into Friday morning. The second will be late Friday
night into Saturday morning. MVFR or lower cigs and vsby will be
possible with each. Thunderstorms may form in between these two
slugs on Friday afternoon, though coverage and intensity are
uncertain. Gradient winds could also gust 20-25 kt Friday
VFR likely Sunday and Monday.
a few gusts to 20 kt have occurred on the southern Maryland waters,
but these are expected to subside with sunrise, with winds
remaining light today. There's a low chance a shower or storm
could reach northern parts of the Bay and upper tidal Potomac
late this afternoon.
Wind fields start increasing late tonight ahead of deepening
storm system and remnants of Cindy. Small Craft Advisory conditions will become
likely by Friday, though a period of moderate to heavy rain
could temper winds a bit in the morning. Winds are expected to
remain elevated through Saturday as the remnants and a cold
front cross the area.
High pressure will most likely result in sub-Small Craft Advisory winds Sunday. A
weak reinforcing cold front may bring gusts to Small Craft Advisory levels
Monday. If so, they will likely subside again Tuesday.
given brief nature of the two round of moderate to heavy rain
and scattered showers/storms in between, areal average storm
total (tonight through saturday) rainfall are generally in the
1-2 inch range, possibly less to the southeast. Thus mmefs
indicates little to no potential of mainstem river flooding. The
main concern will be flash flooding in any areas which receive
repeated and/or prolonged periods of heavy rain.
water levels a few inches above astronomical normals this
morning. These will slowly increase over the next couple of
days. There is the potential for minor flooding by Friday at
sensitive sites, in the gradient flow ahead of a cold front
which would be merged with moisture from the remnants of Cindy.
District of Columbia...none.
Marine...Small Craft Advisory until 8 am EDT this morning for anz533-