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Quantitative Precipitation Forecast (QPF) Discussion
 
(Caution: Version displayed is not the latest version. - Issued 0802Z May 23, 2018)
 
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Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
402 AM EDT Wed May 23 2018


Prelim Day 1, Day 2 and Day 3 QPF Discussion
Valid May 23/1200 UTC thru May 26/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1

...Southeast and Mid Atlantic...

Scattered convection is once again expected along and south of a
southward moving cold front. A few different forcing mechanism are
expected to locally focus convection and produce some higher areal
averaged amounts. The first area of focus will be across eastern
North Carolina where low level convergence along the front should
be strongest. Enough shear here for some organization of
convection near the front. Storm motions should generally be off
to the south, but not all that quick, and some regeneration near
the front may lead to some brief training. The 0z HREF depicts
this scenario well, with moderate probabilities of localized 3"
amounts focused across the area.

Another area of focus will be the lingering low/mid level low over
AL/GA. As expected, thunderstorms have concentrated near the
center of this system early this morning, producing a narrow swath
of heavy rains near the AL/GA border. With diurnal heating expect
convective activity to expand outward from the center as has
happened the past few days. Again these cells will generally be
pulse in nature and short lived. Although continued high PWATs
support brief heavy rates, and wherever cell mergers along
outflows occur, a localized flash flood risk may exist. Will see a
contraction of activity towards the center after sunset over south
GA. Although in general the model consensus seems to suggest the
focus Wednesday night will not be as concentrated/heavy as what is
occurring early this morning.

May also see the terrain of the western Carolinas act as an
initiation focus for storms this afternoon. Although with drier
air moving in from the northwest the signal is not as strong as
previous days. Thus cells may generally be more isolated in nature
and short lived. Nonetheless some very localized heavy rates will
be possible.


...Lower MS Valley...

The Lower MS Valley will be a focus for convection again today. A
weak shortwave trapped underneath the mid level ridge axis will
act as forcing mechanism for storms. Plenty of diurnally driven
instability forecast, and PWATs will remain well above normal.
Storms will be of the pulse variety and will generally be short
lived, however weak mean flow suggests slow cell motion. Will
probably generally see chaotic cell motion with storms forming
along outflows and quickly dissipating. The 0z HREF signal is a
little more robust than yesterday, so anticipate convection will
be a bit more widespread today, resulting in higher overall areal
averaged QPF. Not expecting any organized flash flood threat,
although where some brief cell mergers occur extending rainfall
duration, anticipate we could see a few localized flash flood
concerns this afternoon over AR.


...Northern Rockies into the Plains...

Elevated convection likely to continue into the morning hours
across portions of southern MN and northern IA, with locally heavy
rains possible along a northwest to southeast training axis. This
activity should generally weaken as we go through the morning
hours with the diurnal loss of the low level jet into the front.
Thereafter details with the convective evolution across this
region is low confidence.

Will likely see some storms form in the unstable airmass near the
northward lifting warm front across SD/ND...although at this point
not much of a signal for organized activity. Also will see storms
fire along the dryline over the western Plains by afternoon. This
activity will move eastward into the overnight hours. Given the
instability over the Plains into the overnight, seems reasonable
that some of this dryline convection may survive eastward as a
cluster or two. Suspect the 0z high res guidance are probably too
weak/isolated with this evening/overnight activity, although
confidence in the details is low. One area where there is a bit
more of a high res signal for organized convection is across
eastern NM into west TX. This area does have a few factors going
for it...one being some enhanced upper level divergence in the
left exit region of the subtropical jet, and the other a bit of an
increase in southeasterly 850 mb moisture transport. So will need
to watch this area this evening for some potential locally heavier
rainfall amounts.

A good signal for some 1" areal averaged rainfall across portions
of northwest WY into central MT. This area will see the strongest
dynamics and above normal moisture. Instability will be limited
here, thus generally think we are looking at more of a steady
stratiform rainfall, maybe with a few embedded heavier convective
cores.


...Northwest...

Widespread shower and thunderstorm activity expected across
portions of the northwest in the broad troughing. The focus for
heaviest amounts appears to be across the terrain of northern CA
into the OR Cascades, and portions of central and eastern OR.
Cells will generally be of the pulse variety and short lived,
limiting overall areal averaged rainfall. PWATs will be around
climatological maximum values though, and thus would anticipate
some localized heavy rates. The best chance for some heavier
totals is probably along/near the Cascades, where the terrain will
likely act as forcing mechanism and result in some repeat cell
activity.


Days 2/3...

...Southeast/eastern to central Gulf Coast/FL...

Deep moisture within confluent flow encounters a cold front moving
across the southeast on Thursday. Diurnal heating produces an
areal average QPF max of 1-1.25 inches near the southern
semicircle of the remnant 850 mb circulation in GA.  Elsewhere
expect locally heavy showers in areas of diurnal instability and
sea/lake breeze initiated lift and convergence near the front.

The models also depicted persistent, deep flow of moisture from
the Bahamas into southeast FL
with peak precipitable water values near 2 inches so showery
conditions are expected along the southeast coast. The pattern
favors inland penetration of the sea breeze from the east coast so
peak afternoon convection should occur over interior portions of
the FL peninsula to the west coast. Lapse rates are not as high as
usual due to warm mid level temps, so bands of larger scale lift
are needed to trigger FL convection, and this is the case as both 


QPF was based primarily on the 00z GFS and 00Z ECMWF which came
into better alignment over the central Gulf coast.  The models are
starting to converge a little closer on the evolution of the
offshore gulf cyclonic circulation. The 00Z NAM slowed down its
forward motion to be closer to the ECMWF but is stronger.  Given
the other models have slower forward motion(further offshore), the
idea was to have slower timing on the arrival of heavier showers
than the 00z NAM.


...Pacific Northwest/Great Basin/Northern Rockies...

A closed mid-upper level low off the CA coast through Thu night
drifts slowly east towards CA. Southerly flow ahead of the
circulations increases moisture with 700 mb convergence driving
rain and shower across northeast CA and northwest NV across into
OR. PW increases to around 1 inch ahead of the low over parts of
OR which is 2 standard deviations above normal, so expected
amounts are above normal for May.

On Fri the deep layer circulation drifts onshore in CA with the
upper trough drifting east across OR.
Well defined coupled 700 mb convergence and 300 mb divergence
cross from northern Ca and NV into OR, with better than average
cluster of higher QPF across southern OR as a result. With the
east drift of the upper trough, the peak upper divergence drifts
into the OR Blue Mountains, so a secondary maxima is expected
there Fri/Fri evening.

Manual progs blended the NAM/GFS/UKMET and 12-00z ECMWF QPF.


...Central Plains to the mid-upper MS Valley and upper Great
Lakes...

Activity will focus ahead of a low level front and pre-frontal
moisture plume extending from northern KS across eastern Nebraska,
southeast SD, northwest IA and MN on day 2.  Clusters of showers
and storms are expected again with convergence on the nose of the
low level jet progressing these areas.
Manual progs blended the 00z UKMET/GFS/ECMWF QPF.

On day 3, Fri, the 700 mb trough move east from the northern and
central Plains towards the mid-upper MS Valley.  Lift from the
approaching wave within the low level moisture plume and modest
low level jet maxima supports more showers/storms where
instability is focused. The east drift of the low level jet and
moisture plume and upper trough difluent flow from day 2 brings
the likely targeted areas for shower/thunderstorms development
from southeast KS across MO, near the IA/IL border, WI, and the UP
of Michigan.
QPF was derived by blending the ECMWF, GFS, UKMET, and NAM QPF. 

Chenard/Petersen

Graphics available at www.wpc.ncep.noaa.gov/qpf/qpf2.shtml