Quantitative Precipitation Forecast
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FXUS04 KWBC 220957

Quantitative Precipitation Forecast Discussion
NWS Weather Prediction Center College Park MD
556 AM EDT Tue May 22 2018

Final Day 1, Day 2 and Day 3 QPF Discussion
Valid May 22/1200 UTC thru May 25/1200 UTC
Reference AWIPS Graphics under...Precip Accum - 24hr

Day 1...


The wet pattern will continue today across much of the east. A
moderately unstable airmass, moderately anomalous PWATs, and broad
troughing will support scattered to widespread convective
activity. A few forcing mechanisms within the broad troughing will
help focus convection. Will have stronger synoptic forcing and
wind shear across the central and northern Mid Atlantic, being
closer to the mid level shortwave and upper jet to the north. Thus
would anticipate convection will be a bit more organized here,
probably generally in small linear segments. Will also have whats
left of the backdoor front that is lifting north as a warm front,
which should locally enhance convergence. Instability will drop
off north of the front, which should cap rates there, however
along and south of the front will have ample instability to
produce locally heavy rates across portions of northern VA into
MD. Storms should form near the terrain of WV/VA/MD this afternoon
and progress eastward. Generally expect storms to be progressive
in nature, limiting the overall flash flood threat. Although a
localized threat probably still exists given the saturated
conditions over portions of the central Mid Atlantic. Also likely
to have storms form along the actual cold front further west over
the OH Valley, and move eastward into portions of western PA.
These should weaken after sunset as they head into the terrain.
Will cover both of these areas with a broad Marginal risk. Gave
some thought about going with a Slight Risk across portions of
northern VA into the DC area, as note some higher HREF
probabilities of localized 3" amounts here, and recent HRRR and
Experimental HRRR runs are wetter as well. In the end stuck with
the Marginal given some uncertainty on the exact evolution and the
expected progressive linear nature of storms. However will need to
monitor through the day, as it would not take much cell merging to
cause issues over this saturated area, and based on trends through
the day, an upgrade to a Slight Risk may be needed around the DMV.

Across the southeast a lingering mid level shortwave will once
again act as a focusing mechanism for storms this afternoon. This
feature is currently over southern AL, and is responsible for the
small scale training shower activity over southeast AL. With
diurnal heating, anticipate a broader area of scattered convection
to form around this wave across portions of AL and GA. As with
past days, not expecting this activity to be organized, and cells
should generally be pulse in nature and thus short lived. However
briefly intense rates are likely, and where cells are able to
merge extending rainfall duration, a localized flash flood risk
exists. Activity should diminish after sunset, with a more
localized band of showers likely near the center of the wave
(similar to this morning).

In between these two areas, the mid/upper level focus for
convection is less evident. However seems likely that we will
again see cells form over the terrain of the western Carolinas,
eastern TN and southwest VA with diurnal differential heating.
These cells will tend to drift eastward off the terrain with time.
Normally this activity would not really pose a flash flood risk,
but given the saturated conditions over portions of this area, a
localized threat likely exists once again. May need to watch
northeast GA into western SC and southwest NC a bit closer this
afternoon. Some chance that we see a bit more enhanced cell merger
threat here, with cells moving northward on the periphery of the
wave over AL/GA possibly merging with cells moving east off the
higher terrain. If this were to occur then may see a bit more
focused flash flood risk develop. Confidence is not high on this
though, and will just need to monitor through the day.

A separate weak shortwave over the Lower MS Valley may also act to
focus some convection over portions of AR/LA/MS today. The QPF
signal amongst the 0z HREF member suggest this activity will be
even more isolated and short lived than the activity further east.
Thus while very localized heavy totals are possible, areal
averaged rainfall should not be all that high.

...Central and Northern Rockies into the Plains...

The approach of the mid/upper level trough will increase
convective chances across the Rockies into the Plains. Upper level
flow will become very divergent over WY by this afternoon, which
combined with increasing instability,and the presence of a low
level boundary, will result in scattered convective development.
Instability should be high enough to fully take advantage of the
anomalous PWATs and produce some locally intense rates. Storms
will generally be progressive in nature off to the north, however
could see multiple rounds of storms given the favorable synoptic
environment. Thus at least a localized flash flood risk probably
exists across WY into southern MT. Eventually these cells should
take a turn to the right and move into the northern plains, with
one or more convective clusters likely. Low confidence on how this
evolves, but locally heavy rains are possible into SD/ND as
convection moves across. Storms should generally be progressive,
although one area to watch may be southwest SD. Some signs that
storms that initiate off the Black Hills could merge with
convection moving northeast out of WY/NE. Should this happen a
more focused flash flood risk could develop.

As we head into the overnight hours appears to be a pretty good
setup for an axis of elevated convection across portions eastern
SD into southwest MN and northwest IA. 850 mb moisture transport
increases and persists through the night, with this elevated
activity likely focusing on the nose of this feature. Storm motion
vectors appear to have a large component parallel to this moisture
convergence axis...suggesting that storms may tend to regenerate
and train for a period from northwest to southeast. Would appear
that both instability and PWATs would be high enough to produce
some pretty heavy amounts should this materialize. Still a good
amount of uncertainty though with both the organization of this
activity and the latitude it sets up at. Generally favored an axis
close to the 0z nam nest and ARW2 for the WPC QPF, although likely
will need to make some adjustments to this as things become
clearer by later today. Given the uncertainty and rather dry
antecedent conditions will carry a Marginal Risk for now, although
some chance a Slight Risk could be required on future updates
depending on trends. Either way, something to watch, as certainly
could see some flash flood potential with this setup.

Days 2/3...

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

Deep moisture within confluent flow and a weakening cold front
across the southeast combines with a  remnant circulation now in
Alabama to support bands of showers over the region. Elsewhere,
showers should form 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.

Manual QPF was based on the 00z GFS/ECMWF.  The 00z NAM was an
outlier bringing low pressure closer to the central Gulf coast
with other solutions much further south so the NAM was given no

...Great Basin/Northern Rockies/Northern High Plains...

An opening upper level low and corresponding 700 mb low near the
ID/MT/WY border Wed morning will lift northeast across eastern MT
Tuesday night through Wednesday and into the Canadian Prairies

Instability under the trough will allow convective showers and
0.75 inch PW is above normal, so amounts are higher than you
typically see with a late May system. Showers occur in the area of
low-mid level convergence and ascent near the low, which leads to
reduced coverage/amounts once the low moves into Canada Thu.
Manual progs blended continuity with the GFS/ECMWF/NAM/21z SREF

...Upper Midwest...

A warm front will focus convection on a plume of moisture with the
ECMWF forecasting near 1.5 inches precipitable water at its peak
as it lifts north across MN Wednesday and Thursday.  Once the
front has departed activity will start to focus ahead of a lee
trough in the high Plains, with another plume of enhanced moisture
forecast to form ahead of the trough and approaching cold front
Thu evening in MN. Clusters of showers and storms are expected
again with convergence on the nose of the low level jet
progressing across MN, areas near the SD/IA border, and eastern
Nebraska.  Manual QPF blended the 00z ECMWF/GFS/NAM.

...West Texas/eastern NM/TX-OK panhandles into west KS...

A broad upper trough persists over the southwest on Wed.  The
ECMWF indicates upper divergence maxima Wed crossing central to
eastern NM and then into the TX panhandle. With continuing low
level southerly flow enforcing the existing moist air mass and
trough into enhance low level convergence, widespread
showers/storms are expected again Wed.

Another deepening low approaches CA Thursday.  The trough over NM
lifts out is replaced by a building mid-upper level ridge.
Isolate to scattered showers/storms are possible but with less
coverage due to weakened low level convergence and lift. Manual
QPF blended the NAM/GFS/21z SREF Mean.

...Pacific Northwest/Northwest CA...

Showers are expected on Wed across northwest CA and northern NV
and OR as 700 mb relative humidity remains high across the region
and afternoon instability in an airmass with lapse rates near 7
deg C/KM triggers the development of showers and storms.
Additional showers should form on Thu as moisture remains in place
and a spoke of mid-upper cyclonic vorticity progresses across
northwest CA and then into OR, generating more showers.
The models are in good agreement on showers developing with a
typical spread developing, with the GFS on the wet side and
displaced further east in OR than the other models and the GEFS
Mean.  More weighting was given to the majority cluster, including
the NAM/Canadian Global/ECMWF.  The UKMET was on the dry side and
given less weight, given favorable moisture and lapse rates in


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


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