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Quantitative Precipitation Forecast (QPF) Discussion
(Caution: Version displayed is not the latest version. - Issued 2040Z Apr 22, 2018)
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Abbreviations and acronyms used in this product
Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
440 PM EDT Sun Apr 22 2018

Prelim Day 1, Day 2 and Day 3 QPF Discussion
Valid Apr 23/0000 UTC thru Apr 26/0000 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1

...Lower Ohio Valley---TN Valley--Southern Appalachians into the

There are no significant model differences with the evolution of
the strong closed low moving slowly from the Lower MS Valley into
the TN Valley this period.  A stream of much above average pw
values will continue to be entrained into this
circulation---supporting widespread heavy precipitation potential
in a region of well defined upper difluence ahead of this
circulation.  There is good model qpf agreement with the overall
heavy precip distribution.  One max region expected in the comma
head/deformation precip area to the north of the closed low track
from the TN Valley to the OH Valley---with 1 to 1.5"+ areal
average totals depicted.  A more convective precip max expected
farther to the southeast from eastern Gulf coast into the
Southeast.  Max values across this region likely across the
southern Appalachians from northeast GA----the Upstate of SC into
western NC where strong southeasterly upslope flow will persist
for most of this period. Rainfall totals in excess of 5" likely
across the southern Appalachians.  While one max precip area is
likely over the southern Appalachians---another precip max
possible along the southeast coast in the vicinity of the upper
GA/SC coast where strong onshore in an axis of greater instability
extending north from FL will persist.  Here 2-3"+ totals possible.

...Northern Rockies into the northern High Plains...

Shortwave energy moving through the Pacific Northwest Sunday
afternoon expected to amplify across the Northern Rockies-northern
High Plains region Sunday.  Strengthening upper difluence and low
level upslope north northeasterly flow will support increasing
precipitation  coverage early Monday across portions of the
Northern Rockies into the northern High Plains.  Model consensus
is for moderate to heavy precip totals from far northeast ID---far
northwest to northern  WY and across the southern portions of MT. 
Early spring heavy snows likely across the northern Rockies of
northwest WY into southern MT.  See the latest QPFHSD for
additional winter weather information.

Days 2/3...

...Mid-Atlantic and Ohio Valley to the Northeast...

Widespread rainfall is expected over portions of the Ohio Valley
and Mid-Atlantic states Monday night into Tuesday as a compact
closed low lifts northeastward out of the Tennessee Valley.  The
slow progression of the system, combined with strong inflow off
the Atlantic and divergence aloft should allow for areas of heavy
and potentially excessive rainfall.  There is some spread with the
timing of the upper low lifting out of the Tennessee Valley, but
overall, models have become better defined with highlighting two
separate areas for rainfall amounts to exceed flash flood guidance
values. First, along coastal regions of the Carolinas and a second
area farther west into the southern Appalachians.  The Day 2
Slight Risk of Excessive Rainfall was maintained across these

The upper low will continue to lift northeastward Tuesday night
into Monday, shifting the chance for precipitation into the
northern Mid-Atlantic states and Northeast.  However, the threat
for heavy and excessive rainfall should diminish as the upper low
weakens and gets absorbed into a northern stream trough amplifying
over the Great Lakes Region.  Model spread and confidence in the
exactly how fast the onshore flow and upper forcing weakens
increases beyond 00Z Wednesday, so adjustments were made to the
broad Day 3 Marginal Risk of Excessive Rainfall extending from
coastal North Carolina and Virginia to the Delmarva and southern
New Jersey.      

...Great Plains...

Model guidance has come into much better agreement with the timing
and amplitude of a shortwave trough dropping southeastward from
the northern Rockies to the central Plains Monday night into
Tuesday.  The fast progression of this system should initially
limit any widespread excessive rainfall concerns, but there is a
growing consensus for areas of moderate to heavy rainfall to
develop with the associated cold front while it presses through
the southern Plains Tuesday night into Wednesday.  


Graphics available at