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Quantitative Precipitation Forecast (QPF) Discussion
(Caution: Version displayed is not the latest version. - Issued 0901Z Jun 04, 2018)
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Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
501 AM EDT Mon Jun 04 2018

Prelim Day 1, Day 2 and Day 3 QPF Discussion
Valid Jun 04/1200 UTC thru Jun 07/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr


The elongated, negatively tilted trough nudging into southern
Ontario and the eastern Great Lakes early this morning will become
reinforced by an upper jet streak/additional shortwave energy
entering the trough's base. The result will be a transformation
toward a more positively tilted longwave trough as the low deepens
through the mid-upper levels over southern Ontario-Quebec later
Mon-Mon night. Favorable exit region forcing east of the trough
(upper level divergence and strengthening lower tropospheric s-se
flow and positive moisture/theta-e transport) will favor
additional mod-heavy rainfall along the New England coast, along
the apex of 1.5"+ PWs ahead of the surface occluded front. The
consensus of the 00Z guidance depicted a a decided eastward shift
in the heaviest rainfall during the day 1 period (12Z Mon-12Z
Tue), i.e. mainly offshore Downeast ME while clipping eastern
MA/Cape Cod. While relatively efficient considering the deep
subtropical moisture plume (relatively high wet bulb zero levels
and 500-300 mb layer PW values near 0.25"), the absence of even
elevated instability will be a significant limitation to the short
term rainfall rates, at least with respect to the excessive
rainfall/flash flood potential given the antecedent soil
conditions/current FFG values. As a result, the Marginal risk for
excessive rainfall that was noted in yesterday's day 2 ERO has
been removed.

...Southern Plains...

A compact area of strengthened WAA and deepening elevated
instability early in the period across the southern Plains,
bolstered by the MCV ejecting east of the High Plains, has
fostered an initial round of convection early, as the decaying MCS
will bring moderate areal-average rainfall totals to central and
north-central TX later this morning. Following round 1, lingering
MCV energy and outflow boundaries along with the stationary
synoptic front will favor additional convective development this
afternoon-evening -- especially toward the mid-upper TX coast
where robust deep-layer instability (model mixed layer CAPES
2500-3500+ j/kg) will be present. The model spread however is
quite high with respect to the convection evolution (and QPF)
later this afternoon and evening, though the solutions do begin to
align somewhat more favorably late Mon evening and overnight
(especially after 06Z) as increasing low level southwesterly flow
(30-40 kts at 850 mb) supports another compact MCS from
south-central OK, across the Red River and into North TX and
western portions of the ARKLATEX. PW values are progged to spike
aoa 1.75" Monday night along the path of this MCS, with the models
(especially high-res CAMs) likely simulating the mesoscale
environment to some degree. In terms of the QPF, given the
disparity among the operational model runs (global and high-res),
WPC once again leaned toward the means, particularly the HREF and
NBM high-res means. Individual CAMs such as the WRF-ARW, ARW2.
NMMB, NSSL-WRF, and NAM CONUS-Nest all show spotty areas of 24
hour rainfall totals of 3-5+ inches, the bulk of which would
arrive late Monday night via the next MCS. Despite the relatively
high FFG initially, the prospects of at least some reduction from
rains early Mon, along with the potential for fairly prodigious
short-term rainfall rates Mon night-early Tue with the MCS, would
continue to warrant a Marginal risk area in the new day 1 ERO
(similar to the day 2 outlook from Sun).

Day 2

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

Low pressure centered over Quebec opens as it shifts east across
New England through Tuesday night. Light to moderate rainfall in
sub one inch PW across New England. A cold front will progress
south from the wave with a likely narrow swath of rainfall from
the OH Valley to the Mid-Atlantic. One inch PW ahead of the front
should allow for localized heavy rainfall in both the central
Appalachians upslope areas and near the coast where bay/sea
breezes could locally add moisture.

...Northern Plains...

Gulf-sourced moisture will converge over ND where surface low
pressure crosses as a shortwave trough lifts over the central
CONUS ridge from MT into the Canadian Prairies. 1.25 inch PW is 2
standard deviations above normal. Ample instability with allow
convective development though progressive/cold pool driven
activity is expected in deep layer westerly flow. Therefore a
marginal risk of excessive rainfall was maintained over eastern ND
into northwestern MN.

...Gulf Coast...

The cold front that has progressed south across the county will
stall over the northern Gulf of Mexico through midweek. PW
anomalies along the front are expected to be 1 to 1.5 standard
deviations through Tuesday night. Mean layer flow along the
orientation of the front and ample instability should allow
repeating thunderstorm development, though most of this will be
offshore. Will need to monitor southern LA and the FL peninsula
for areas of repeat convective thunderstorms.

Day 3

...North-Central Plains to Upper MS Valley...

Ongoing MCS activity is likely in the southerly low level flow
streaming up the Plains from the Gulf. Heavy rain is a concern
with PW around 1.5 standard deviations above normal over SD/NE and
east into IA Wednesday. However, the mesoscale properties
associated with this development make for a difficult location
forecast. No excessive rain risk is raised as of now.

...Louisiana and Florida...

The lingering cold front over the Gulf will continue to extend
from LA to Central FL---focusing an axis of tropical moisture
along and to its south through midweek. Heavy rainfall is likely
though the PW anomaly diminishes to near normal Wednesday, so no
excessive rain risk is raised at this time.


Graphics available at