Prognostic Meteorological Discussion
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FXUS01 KWBC 211955

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
255 PM EST Wed Feb 21 2018

Valid 00Z Thu Feb 22 2018 - 00Z Sat Feb 24 2018

...Cold and unsettled weather is expected over the West including
some heavy snow over the Rockies...

...Additional heavy rainfall and widespread concerns for flooding
will persist across parts of the southern Plains, lower
Mississippi Valley and the Ohio Valley...

...Warm temperatures are generally expected to continue across a
large portion of the East...

The Western U.S. will mainly be cold, but also unsettled through
the end of the week, as a series of impulses dropping south across
western North America help to reinforce a broad upper level trough
of low pressure. This will maintain below normal temperatures with
most of the West seeing high temperatures about 10 to 20 degrees
below normal. This energy will also bring the threat for heavy
snow across parts of northern California, the Great Basin and the
central Rockies as the energy traverses the interior of the West.

Unfortunately, these impulses of energy ejecting down across the
West and then out into the Plains will be the primary drivers of
multiple rounds of heavy rainfall farther off to the east. This
will include portions of the southern Plains, lower Mississippi
Valley and Ohio Valley as multiple waves of low pressure eject
northeast up along a frontal zone draped across the region. This
additional rainfall will be largely falling on saturated ground
from recent heavy rain over the last several days and going back
to last week in some cases. Therefore, the threat for additional
flash flooding and main stem river flooding will become enhanced
through the remainder of the week.

Aside from the focus for heavy rainfall, the same aforementioned
front is marking the difference from an abnormally warm airmass in
place across much of the East where temperatures are reaching as
high as the 70s and 80s, and a very cold airmass that has nosed
south down across much of the Plains where temperatures are
locally below freezing as much as 10 to 20 degrees below normal.

Over the next couple of days, some of the warm air across
especially the Northeast and parts of the Mid-Atlantic region will
be tempered by a modestly colder airmass behind a frontal passage
later tonight, but this boundary will stall out from the lower
Mississippi Valley to across the Tennessee Valley and into the
Mid-Atlantic region on Thursday before then gradually beginning to
lift back north as a warm front by Friday as a new area of low
pressure lifts up across the Midwest. This will keep the Southeast
U.S. in particular quite warm with much above normal temperatures
through the end of the week.

Regarding the threat of winter weather, there will be the approach
of a wave of low pressure from the Tennessee Valley on Thursday
that will actually interact with sufficient cold air over parts of
the upper Ohio Valley, far northern Mid-Atlantic and interior of
the Northeast for a stripe of some accumulating snow and ice.
Meanwhile, there will also be a swath of accumulating snow and ice
across portions of especially the central Plains, Midwest and
Great Lakes for Thursday and Friday as low pressure rides
northeast up across the region.


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