Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1100 AM EST Sun Feb 17 2019
Valid 12Z Wed Feb 20 2019 - 12Z Sun Feb 24 2019
...Multi-day heavy rain threat for Lower Mississippi/Tennessee
Valleys to the Southern/Central Appalachians and snow/ice for the
Mid-Atlantic to Southern New England...
...Cold weather persists in the western to central U.S., with
unsettled weather focusing into the Southwest...
...Overview...
Persistent western U.S. mid-upper level troughing will be
reinforced during the beginning of the medium range period by
shots of renewed energy. The trough will eventually move eastward
into the central U.S. by next weekend. This feature plus an
unyielding Bahamas upper ridge creates a favorable pattern for a
multi-day heavy rain threat this week from the lower MS/TN Valleys
to the Southern/Central Appalachians and a snow/ice threat for
parts of the Mid-Atlantic and the Northeast Wed-Thu.
...Guidance/Predictability Assessment and Preferences...
Guidance is in general agreement for days 3-4 of the medium range
period regarding the pattern of troughing across the western U.S.
and a leading trough moving across the north central U.S. on Wed
and into the Northeast on Thu, overtop the southeastern ridge. A
multi-model blend of deterministic models was used for this time
frame. By days 4-5, while model agreement across the CONUS is
reasonably good, the 00Z deterministic ECMWF develops an upper low
in the Gulf of Alaska/northern Pacific that is much stronger than
other models and its mean, due to its handling of energy upstream
in the northern Pacific. This cutoff low (where other models have
a trough) progresses southward through the rest of the period and
affects the western U.S. Thus leaned away from the 00Z
deterministic ECMWF for this feature. By the end of the forecast
period, a surface low is expected to develop in the
central/eastern U.S. as the main trough in the western U.S. ejects
out into the central U.S. The most recent GFS runs seemed too
suppressed with this surface low, considering the strength of the
upper vort max and the reasonable forecast confidence. A blend of
the means was used by the end of the period, along with the
deterministic CMC and the parallel FV3 GFS to maintain details in
strength.
...Weather Highlights/Hazards...
An prolonged excessive rainfall threat from the lower MS/TN
Valleys to the Srn/Central Appalachians will be one main story
this week. Deep layered and highly anomalously moist southwest
flow channeled from the Southern Plains to the East will act to
sustain a Southeast upper ridge and fuel multi-day threats of
heavy rain/training. On the northern edge, a cold surface high and
cold air damming in the Mid-Atlantic region is expected, leading
to frozen precipitation. Potentially heavy snow and ice is a
possibility especially in the northern Mid-Atlantic into southern
New England Wed-Thu.
With anomalously low heights in the western U.S., much below
average temperatures will prevail for the western and central
U.S., while ridging in the Southeast will lead to above average
temperatures. A main low is progged to dig from the Pacific
Northwest to California Wed/Thu to the Southwest by Fri along with
enhanced precipitation, including higher elevation heavy snow. The
main system is expected to eject northeastward across the central
and east-central U.S. next weekend.
Tate/Schichtel
WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation and winter weather outlook
probabilities are found at:
https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4