Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

fxus61 kakq 212127 

Area forecast discussion
National Weather Service Wakefield Virginia
427 PM EST Sat Jan 21 2017

low pressure will track across the Carolinas tonight. A second
area of low pressure over the nations mid section will intensify
as it slowly tracks east across the mid Atlantic region Sunday
and Monday. The intense low will move off the New England coast


Near term /until 6 am Sunday morning/...
latest msas still shows the region wedged in a rather stable airmass
along with lingering fog and stratus ahead of the next potent S/W
approaching from the SW. Models have this moisture overspreading
the region this evening then continuing to move NE through the
overnight hours along the coast but tapers off to drizzle/fog
across the west after 06z. Forecast challenge is just how far
north the associated warm front gets. Data suggests it gets as
far north as northern NC. Best lift/support for pcpn remains
over the south where categorical pops will be maintained. MDT to
locally hvy downpours will be possible for a few hours. High
chc to likely pops far north. Best instability progged south of
the front but data does support some elevated thunder this
evening mainly across NC. Lows mid 40s north to lwr 50s south.


Short term /6 am Sunday morning through Tuesday/...
first S/W exits to the NE Sunday morning with the fa in between
systems ahead of the stronger system approaching from the SW late.
Plenty of low level moisture in the tsctns should result in another
morning with areas of drizzle/fog and possible low vsbys. Moisture
quickly returns/overspreads the fa after 18z as strong sfc low
pressure and shortwave energy aloft approach from the west.
Increasing pw's will result in moderate to locally heavy rainfall.
Categorical pops all areas after 18z. Warm frontal boundary progged
to drift north to near the NC-Virginia border so kept chc thunder across
southern half of fa during the afternoon with decent shear as well.
Storm Prediction Center has slght risk of severe up to the Albemarle Sound with a marginal
risk across srn Virginia. Appears damaging winds are the main threat.
Threat will be later in the day into the evening hours. Highs in the
mid-upr 50s north to mid-upr 60s south.

NAM a bit faster than the GFS and much faster than the European model (ecmwf)
with the movement of the stacked low, but the model trends are
now more progressive than before. Deep Atlantic moisture feed
results in MDT to locally downpours Sunday night tracking south
to north along with thunder chcs during the evening. Categorical
pops all areas with lows in the upr 40s to lwr 50s. Quantitative precipitation forecast averages
between 1 to 2 inches with locally higher amounts in any banded
convection that may occur.

Sfc triple point progged to move north of the area Monday morning
with the trailing upr level low crossing the region during the
afternoon. Significant dry slot progged to cut off the deep moisture
feed. Main change to mondays forecast was to lower pops to chc most
areas except low end likely across the west. Tstcns also show that
some breaks in overcast are possible leading to some instab after 18z. Have
added chc thunder Monday afternoon to account for this. Well above
normal temps continue with highs ranging from the mid-upr 50s nwrn
most zones to the upr 60s across the se.

System continues to track NE Monday night but GFS has yet another
vort lobe spoke rotating arnd the main low. Will keep low chc pops
in the grids for now with this feature. Lows in the 40s.

Finally get a break Tuesday as winds become northwest and dry out the
column. Expect skies to become pt to mstly sunny. Temps remain above
normal with highs in the 50s.


Long term /Tuesday night through Saturday/...
overall, the models are in good agreement in the extended period
with only a few minor differences on the timing of the frontal
passage on Wed night into Thursday. But those differences are
minor and the 12z GFS has moved closer to the 00z European model (ecmwf).

In the wake of the storm that will move through by Tuesday, a
shortwave ridge builds in tues night into Wednesday. This will
allow for west to SW flow and should be a mild dry period. However,
the upper level pattern will begin to flip with a broad trough
developing over the eastern half of the country for the second
half of the week. This trough will enter the region Wed night
into Thursday as a sfc cold front slides into the area. This
front will be lacking moisture so have limited the pops to
slight chance to chance range. The best opportunity for rain
looks to be in the southeastern part of the area where a little
additional Atlantic moisture may be available. The timing on the
front is still a little up in the air. The European model (ecmwf) moves the
boundary through more quickly, while the GFS develops a wave and
is slower. But the trend has been toward the European model (ecmwf) timing so
focused the highest chance on Thursday. Will also need to watch
the timing on the cold air arrival as well as this could impact
the pcpn type, but for now don't think the cold air will arrive
until after any pcpn is gone.

For Thursday night into Saturday, the region will be dominated
by strong northwest flow which will drop temperatures back down into
the 40s for highs and lows in the upper 20s to low 30s. Have
kept the fcst dry too, but will have to monitor shortwaves
rotating through the upper trough as with this polar airmass, if
one of these waves amplifies a bit, some snow showers would not
be out of the realm of possibility.


Aviation /21z Saturday through Thursday/...
widespread IFR ceilings expected to persist through the entire
taf period. With no mixing and the warm front well to south
expect IFR ceilings to persist through the afternoon and
overnight. A weak area of low pressure will move acrs NC and southeast
Virginia later tngt thru sun. This will bring a decent wave of rain to
spread from SW to NE across the entire region from 01z to 12z.
Also expect some low level wind shear in areas across southeast Virginia and NE NC overnight.

Outlook...periods of rain, moderate to heavy at times,
Sunday through Monday, will be accompanied by IFR conditions. As
low pressure moves off to the northeast...dry weather and a
clearing sky is indicated for Tuesday and Wednesday.


weak high pressure still is lingering over the waters this
afternoon and with southerly flow aloft pushing moisture
northward, seeing lots of low clouds and fog. This pattern will
persist for a few more hours, before an area of low pressure
move northward off the North Carolina coast. This will help to
increase the winds and also with some rain showers will probably
lead to visibilities increasing a little more between 1 - 3 sm.
This low will track off the coast on Sunday as more showers
enter the region ahead of warm front and a much stronger area of
low pressure that will be moving into the Tennessee River valley
by Sunday afternoon. Expect easterly winds to increase during
the day, but to remain below sca levels.

The strong low will slide over the central Appalachians Sunday
night while a new areas of high pressure, centered over eastern
Canada builds down the coast. The pressure gradient will tighten
off the New Jersey/Delaware coast and will extend southward into the coastal
waters off the Delmarva. This will allow winds to increase to
sca and possibly some gust to gale in the northern coastal zone.
At this time have just raised the sca flags as the gale
conditions are not a certainty and it is still about 36 hours

The center of the low pressure system slides over the mid-
Atlantic states on Monday and the gradient relaxes so expect the
winds to diminish quickly Monday afternoon. But the seas will
remain up in the coastal zones for some time longer. The Small Craft Advisory
will probably need to be extended further in time, probably into
Tuesday for the coastal waters. The Bay and inland waters will
remain below sca conditions through the event, only getting
close to sca conditions Sunday night into early Monday with
winds between 15 to 17 kt.

The low pressure area will slide north of the region on Tuesday
and high pressure over the southeastern US will build into the
area bringing a period of dry and tranquil weather to the area.
The next cold front will arrive on early Thursday. With the
colder air and stronger winds slower to arrive on Friday.


Akq watches/warnings/advisories...
Marine...Small Craft Advisory from 1 am to 4 PM EST Monday for anz652-
Small Craft Advisory from 7 PM Sunday to 4 PM EST Monday for



National Weather Service Glossary of Abbreviations