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March Obs/Disco Thread

arlwx

By arlwx,
in Mid Atlantic

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arlwx

arlwx

Posted
Posted

According to LWX, March IS coming in like a lion...


Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
401 AM EST Wed Mar 1 2017

.SYNOPSIS...

A strong cold front will pass through the area late today and
tonight. After a brief period of high pressure Thursday, an
area of weak low pressure will cross the region Friday. High
pressure returns for the weekend.

&&

.NEAR TERM /TODAY/...

This March is coming in as a lion.

As of 3am, a sprawling low is across the Midwest states with a 997mb
surface low over southern Lake Michigan with a 1028mb surface high
over Bermuda. Two notable thunderstorm clusters are across the
Midwest - a leading area over the central Ohio Valley associated
with a southern stream shortwave and prefrontal line which stretches
from Detroit to Springfield, MO. Warm air is over the Mid-Atlantic
with temperatures generally mid 50s (near high min records, see
climate section below). Valley fog is occuring in the Potomac
highlands and is expected to dissipate through sunrise as rain
enters the area.

The Ohio Valley activity is shifting ENE and is on track to graze
the NWrn zones (including Cumberland) this morning. A few hundred
MUCAPE may allow isolated thunder. The associated shortwave moves NE
across central and western portions of the CWA late morning into the
afternoon. Associated gusty showers and thunderstorms look to take
advantage of midday heating and produce lines/segments across the
Balt-Wash metro. However, the forcing looks to become parallel to
the flow and limit potential for southern MD.

The actual cold front crosses the area late this evening through
midnight. Expect another gusty convective line on the front with
thunder possible. Wind shifts NW with late night strong winds across
the area. Wind Advisory potential late tonight with gusts 40 to 50
mph possible across the CWA in cold air advection.

&&

.SHORT TERM /TONIGHT THROUGH FRIDAY/...

High pressure builds Thursday with a deep low over the Canadian
maritimes and the surface high over OK. Downsloping flow makes for
max temps around 50F.

Late Thursday night into Friday an Alberta clipper scoots across the
area. 00Z GFS/ECMWF prog the surface low across DC while the NAM has
been farther north/over PA. The 06Z NAM tracks the low over southern
PA while the 00Z was over northern PA. The upper trough axis shifts
east across the area late morning through midday. Instability under
this low looks to produce snow streamers/showers over the area
during daylight hours Friday. Guidance consensus max temps still is
in the 40s across the area to around 50F well south of DC in gusty
NWly flow. However, with the 850mb temps around -10C, wet bulb zero
temps are less than 1000ft (a critical level for snow/rain).
Therefore, went for snow exclusively for NWrn suburbs and rain or
snow metro areas and south.

Downsloping plays an important role in snow streamers in NWly flow.
Elevation increases from the Laurel Ridge of PA south along the
Allegheny Front of MD/WV. Therefore, accumulating snow chances
generally decrease south from the Mason-Dixon line due to increased
downsloping. However, the narrow bands that develop can produce
localized accumulating snow wherever they set up. Superfreezing
temperatures will limit impact.
(snip)

.CLIMATE...

Here is a list of record daily warm temperatures for March 1st (Wednesday).

Site   Record High   Record Warm Low
DCA    80 (1976)     57 (1910)
BWI    80 (1972)     53 (1910)
IAD    77 (1972)     43 (1997/1987/1976)

Here are updated February and Winter records. It appears that this will
go down as the warmest February on record at all three major
airports.

Warmest Februaries (average temperature)
   DCA           BWI           IAD
1. 47.7 (2017)   44.2 (2017)   44.8 (2017)
2. 46.9 (1976)*  44.0 (1976)*  42.1 (1990)
3. 45.2 (1990)   43.9 (1949)   41.1 (1976)*
4. 44.7 (1997)   43.3 (1890)   41.0 (1998)
5. 44.3 (2012)*  42.7 (1932)   40.9 (2012)*
6. 43.9 (1949)   42.6 (1909)   40.5 (1997)

*Leap year

Overall winter

Washington had the third warmest winter on record,
Baltimore will have a top 10 warmest winter.
This is the warmest winter on record for Dulles.

Warmest Winters (Dec 1-Feb 28/29)
   DCA              BWI              IAD
1. 44.7 (1931-32)   45.3 (1931-32)   40.0 (2011-12)
2. 44.3 (1889-90)   44.4 (1889-90)   39.7 (2001-02)
3. 43.3 (2011-12)   42.4 (1948-49)   39.4 (1997-98)
4. 43.2 (2001-02)   41.9 (1949-50)   39.0 (2015-16)
5. 42.8 (1949-50)   41.3 (1879-80)   38.3 (1990-91)

Winter 2016-17 (through the 28th)
DCA: 43.9           BWI: 40.5        IAD: 40.7

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory until 11 AM EST this morning for
     ANZ530>543.
     Gale Warning from 11 AM this morning to 10 AM EST Thursday for
     ANZ530>543.

&&

$$
PRODUCTS...BAJ/WOODY!

 

 

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arlwx

arlwx

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LWX is less confident about Friday's clipper.

(snip)

The Alberta clipper tonight is weaker from previous day in 00Z
consensus guidance. That said, any resemblance to a surface low is
progged between DC and the Mason-Dixon line. Therefore, the threat
is for a band of light snow in this area late tonight. Went for 50
to 60 PoPs north of the DC metro, stretching east to Baltimore along
with light

Friday, the upper trough axis shifts east across the area late
morning/midday. Instability under this low looks to produce snow
streamers/showers over the area during daylight hours Friday.
Guidance consensus max temps generally low to mid 40s, but with
the steep lapse rates, snow is the expected p-type.

Downsloping plays an important role in snow streamers in NWly flow.
Elevation increases from the Laurel Ridge of PA south along the
Allegheny Front of MD/WV. Therefore, accumulating snow chances
generally decrease south from the Mason-Dixon line due to increased
downsloping. However, the narrow bands that develop can produce
localized accumulating snow wherever they set up. Superfreezing
temperatures will limit impact.

 

 

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yoda

yoda

Posted
Posted

Never heard of Froude numbers before 

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
247 PM EST Thu Mar 2 2017

.SYNOPSIS...A fast moving area of low pressure will move across the region
early Friday. High pressure returns for the weekend. A cold front
is expected to push through the Mid Atlantic during the early
part of next week.

&&

.NEAR TERM /TONIGHT THROUGH FRIDAY/...Shortwave energy will
move across the area tonight and Fri morning as broad trough
across the Great Lks amplify. Expect snow showers to increase in
coverage/intensity over the Potomac highlands. Fcst Froude
number from 12Z NAM are well over 1 indicating the snow showers
will be able to cross the mtns. Snow squall parameter, low-level
instability, and Froude numbers over 10 indicate potential for
snow squalls to reach the Chesapeake Bay during the morning and
early afternoon. A little dusting is possible in some areas
mainly on grassy surfaces. It will also be breezy with gusts in
the 25-30 mph.
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Round Hill WX

Round Hill WX

Posted
Posted
28 minutes ago, yoda said:

Never heard of Froude numbers before 


Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
247 PM EST Thu Mar 2 2017

.SYNOPSIS...A fast moving area of low pressure will move across the region
early Friday. High pressure returns for the weekend. A cold front
is expected to push through the Mid Atlantic during the early
part of next week.

&&

.NEAR TERM /TONIGHT THROUGH FRIDAY/...Shortwave energy will
move across the area tonight and Fri morning as broad trough
across the Great Lks amplify. Expect snow showers to increase in
coverage/intensity over the Potomac highlands. Fcst Froude
number from 12Z NAM are well over 1 indicating the snow showers
will be able to cross the mtns. Snow squall parameter, low-level
instability, and Froude numbers over 10 indicate potential for
snow squalls to reach the Chesapeake Bay during the morning and
early afternoon. A little dusting is possible in some areas
mainly on grassy surfaces. It will also be breezy with gusts in
the 25-30 mph.

Froude numbers are used to indicate the ease to which precipitation can overcome a barrier(mountains). Anything over 1 shows that airflow over the mountains is unblocked and can flow freely. Thus precipitation, if available will be able to survive the trip over the mountains. 

NWS Burlington uses Froude number frequently as they are useful with the Green and Adirondack mountains and their effect on the Champlain Valley. 

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