Scientific Forecaster Discussion
Area forecast discussion...updated
National Weather Service Baltimore Maryland/Washington District of Columbia
116 PM EST Friday Dec 6 2013
a cold front will pass through the area this morning. A wave of low
pressure will develop along the front...bringing unsettled
conditions through tonight. High pressure will build overhead
Saturday...but low pressure will impact the area Sunday into Monday
with wintry precipitation. An upper-level disturbance will pass
through the area Tuesday and Canadian high pressure will build
overhead during the middle of next week.
Near term /through tonight/...
headline update...1). Small Craft Advisory expired for the Bay from the morning hours.
Another Small Craft Advisory in effect from late tonight through Sat evening. 2). Winter
Storm Watch now in effect for the counties west of the northern Virginia Blue
Ridge/Maryland catoctins...including the lower foothills include
Nelson/albe/Greene co's. See wbcwswlwx for further details. The
watch is in effect from around sunrise Sunday morning through middle-morning
Monday. Different phases of this system will make for a complex
event...W/ snow on the initial push Sunday changing to a wintry
mix as times GOES on into the overnight hours late sun into early
Monday. Further diagnostics of this system will be in the afternoon afd
issuance...this is the intermediate disc update for headline
From previous disc...
after a heavier batch of moderate rain associated west/ a low level
jet maximum has mostly moved through...periods of light rain will
continue into the middle afternoon hours...then a couple hours of a lull
period late afternoon into early evening.
A wave of low pressure will develop along the front this evening
before eventually passing off to our southeast late tonight. More
rain is expected across the entire area. A soaking rain is
expected with rainfall amounts through tonight ranging from one
half to one inch across most areas...but over one inch across
portions of the Potomac Highlands. A dry northwest flow will work
its way into the area after midnight...causing precipitation to
taper off during this time.
Will continue to watch low-level cold air moving in place later this
afternoon through tonight. There will be a layer of below freezing
temperatures that may move into portions of the Allegheny Highlands during
this time. With warm air aloft...that may cause a period of freezing
rain across these areas. The best chance for that to happen will be
this afternoon and evening. Precipitation will end as a little snow
and sleet after midnight...but snow and sleet accumulation will be
light. Have not issued an advisory at this time since it will be
marginal and confined to a small layer between around 1000 and 3000
feet above sea level. Will have to monitor temperatures upstream
throughout the day to see if a Winter Weather Advisory is
Short term /Saturday through Saturday night/...
Canadian high pressure will build toward the region as the cold
front continues to move way off to our south and east. A dry
northwest flow will continue to usher in drier conditions along with
some sunshine. Any leftover snow showers along the Allegheny
Highlands should end during the morning and any leftover showers
across extreme southern Maryland will also dissipate during the
Despite dry conditions and increasing sunshine...temperatures will
be much colder compared to recent days due to the cold northwest
flow. Maximum temperatures will range from the upper 20s in the mountains to
the lower and middle 40s in central Virginia.
Canadian high pressure will settle overhead Saturday night...causing
cold conditions. Min temperatures will be in the 20s across most areas. A
storm system will move out of The Rockies into the central Continental U.S.
Saturday night. High and middle-level clouds will increase ahead of
this system. A southwest flow aloft ahead of this system will
cause plenty of warm and moist air from the Gulf of Mexico to
overrun the low-level cold air in place. This will cause
precipitation to break out over the Tennessee Valley Saturday night.
This precipitation will spread to our southwestern areas toward day
break. Temperatures will be cold enough to support snow...and snow
may begin toward sunrise across portions of central Virginia.
Elsewhere...wintry precipitation will likely hold off until Sunday.
Long term /Sunday through Thursday/...
sun-Monday still looking like a complicated forecast...but height fields
depicting remarkable agreement in overall pttn. Thermal pttn h9-7
still a bit uncertain...which will be the key to this forecast.
The day will start Sunday west/ a hipres ridge /sfc pressure 1036 mb/ over
NY/PA...supplying plenty of low level cold air. Warm air advection will get cranking across
the MS/Ohio valleys...providing a warm air intrusion-- the depth and
extent of which still tbd. Am thinking that evap cooling will assist
in starting precipitation as snow...eventually mixing west/ pl/fzra. As surface ridge
becomes entrenched along the aplchns and warm air advection continues h9-7...precipitation will
become incrsgly freezing rain based...changing over to rain as the marine influence
strengthens. Am not willing to specifically time transitions as
driving forces subtle...and still subject to change. There are no significant
synoptic features involved aside from surface hipres...purely middle-upper level
dynamics. Regardless of specifics...the Sunday into Ely Monday period
looking messy. All interests should monitor future forecasts. As the WV of
precipitation exits Monday...the gnrl theme should pt to mainly rain as temperatures do
get above freezing.
GFS/European model (ecmwf) part ways at this point. Euro placing greater emphasis on
cold surface high...GFS leading toward anthr WV riding alng stalled boundary
Tuesday. Vorticity dropping into Great Lakes should bump boundary eastward a bit...and
therefore will be leaning forecast on the drier side. Would consider extended
forecast confidence a little low based on this spread.
Aviation /18z Friday through Wednesday/...
ceilings will bounce back-and-forth between IFR and low-end MVFR late
this morning west/ another passage of widespread moderate rain. After
this batch Post around 18z...brief improvement before another wave
of precipitation later this evening.
Rain will continue for most of tonight and ceilings may drop to IFR
levels late this afternoon through much of tonight as a wave of low
pressure passes by to the south. North-northwest winds will dry
things out late tonight into Saturday morning...causing ceilings to
improve to VFR levels during this time. VFR conditions will persist
North-northwest winds will gust around 20 knots for a few hours
behind the front this morning...but a weaker gradient will cause
gusty winds to diminish for this afternoon and evening. Another
pressure surge is expected behind the wave of low pressure late
tonight through Saturday and north-northwest winds will gust around
20 knots. Winds will diminish Saturday night.
Numerous flgt restrictions sun into Monday in wintry weather. Precipitation should start as
snow...then mix/change to pl/fzra...and eventually rain. Dca/BWI/mtn will
complete the process quickest...mrb will be the slowest. Some of these
restrictions may linger into the Monday morning push. Although temperatures will
slowly rise above freezing Monday...still xpctg low ceilings. Gradual impvmnt will
come Monday ngt-Tue.
a pressure surge behind a cold front that moved over the waters a
few hours ago...will cause north-northwest winds to gust around 20
knots. A Small Craft Advisory remains in effect.
A wave of low pressure will pass by to the south this evening and
another pressure surge is expected behind the wave late tonight
through Saturday. A Small Craft Advisory is in effect for the waters
during this time. North-northwest winds will continue Saturday night
and a Small Craft Advisory may be needed for portions of the
Maryland Chesapeake Bay and lower tidal Potomac River.
A period of wintry weather expeceted sun-Sun night. However...water temperatures should assist
in transitioning precipitation to rain...especially on the open waters of the Bay.
Mixing through this period should be poor. Northerly winds will gradually become Ely...
generally at or below 10 knots. Winds will attempt to veer southeasterly on Monday...it likely will be
a slow process.
As the storm system progresses into the Atlantic...westerly winds will take
hold. Should have some gusty winds at that pt.
an unsettled weather period is in store for the middle-Atlantic region.
Forecast rainfall amounts through Friday night are lower than previously
expected...generally on the order of 0.50-1 inch...which creates
only minor rises in rivers.
More precipitation is projected over the region Sunday into
Monday...and this will likely be heavier than the first round.
Naefs and gefs mean rainfall for both events combined is between
two and three inches. While the latest models have backed off on
quantitative precipitation forecast and river stage forecasts a bit...looking like only a low
potential for some flooding issues on small streams and/or
mainstem rivers Sunday through early next week. If frozen precipitation
falls...the melting of it factors into the equation too. There is
still a great deal of uncertainty regarding where the heaviest
precipitation sets up...and whether it stays in one place for a
tidal anomalies remain around one half to one foot above normal this
morning. However...north-northwest winds behind a cold front will
likely cause anomalies to decrease this afternoon. Therefore...
coastal flooding is not expected at this time.
District of Columbia...none.
Maryland...Winter Storm Watch from Sunday morning through Monday morning
Virginia...Winter Storm Watch from Sunday morning through Monday morning
WV...Winter Storm Watch from Sunday morning through Monday morning
Marine...Small Craft Advisory from midnight tonight to 6 PM EST
Saturday for anz530>543.