Prognostic Meteorological Discussion
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FXUS02 KWBC 241550
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1149 AM EDT Thu May 24 2018

Valid 12Z Sun May 27 2018 - 12Z Thu May 31 2018

...Confidence increasing in potential heavy rainfall event for
portions of the Gulf Coast and Southeastern states this weekend
into early next week...


...Overview/Synoptic Assessment...

Blocky flow pattern expected to persist across North America
through the medium range, with persistent and relatively
slow-propagating height anomalies. Negative height anomalies are
expected to persist across northeast Canada and Greenland, and
subtropical ridge remaining the dominant features off the U.S.
Eastern Seaboard. A cutoff upper low initially across the Great
Basin will be slow to move east during the Sun-Mon time period as
it remains trapped beneath broad upper ridging across the Pacific
Northwest/southwestern Canada. As heights fall to the north of the
feature into early to mid next week, increasing influence of
westerlies will allow the system to gradually move east into the
northern plains/Upper Midwest. The lack of any significant
influence of the westerlies early in the period across the
southern tier will allow low pressure in the Gulf of Mexico to
move northward toward the mouth of the Mississippi River by Mon,
with perhaps a slightly increased northward pace by Wed-Thu in
response to the shortwave moving across the northern
plains/Midwest. Finally, another trough is expected to reach the
West Coast by Tue-Wed, undercutting a ridge axis across western
Canada once again.


...Guidance and Predictability Assessment...

Model consensus with respect to low pressure across the northern
Gulf of Mexico on Sun has improved a bit over the past day. The
06Z GFS made a significant shift farther west, and now looks much
more like the ECMWF/CMC. Additionally, the number of GEFS members
taking the low across the Florida Peninsula has dwindles to nearly
zero. The biggest change among this guidance with respect to this
feature is by next Wed-Thu when the consensus now carries the
system farther north a bit more quickly, into the Tennessee valley
by Thu morning. This appears to be in response to fairly minor
changes with the shortwave moving from the Rockies into the
northern plains, with most guidance showing lower heights compared
to 24 hours ago, and perhaps stronger mid/upper-level flow digging
a bit farther south across the central U.S., resulting in a
greater degree of interaction between the two features. As of this
morning, the eventual northern plains shortwave remains several
hundred miles off the West Coast, and out of the North American
RAOB network, thus would expect that any potential shifts among
the guidance may not be over. Nonetheless, guidance spread
surrounding this Rockies to Upper Midwest upper low/shortwave is
relatively small, with fairly minor timing/amplitude differences.
With additional trough expected to reach the West Coast by next
Tue-Wed, the 00Z ECMWF/06Z GFS showed similar timing/amplitude,
and the CMC was also similar but slightly slower. Consensus
appears good that significant height falls should reach the
Northwest by next Wed-Thu as this trough moves inland.

The WPC forecast was based on a heavily deterministic blend,
including the 00Z ECMWF/CMC/06Z GFS through much of the medium
range. Some weight was placed toward the 00Z ECENS/06Z GEFS means
by by days 6-7 (Wed-Thu) to account for increasing spread,
although model spread for that time frame seems a bit below
average.


...Weather/Threats Highlights...

Confidence is increasing that the Gulf of Mexico low pressure,
regardless of any potential tropical or subtropical development
(see NHC products for more details), will bring the potential for
a widespread, multi-day heavy rainfall event to portions of the
Gulf Coast and Southeast states. Deterministic ECMWF and GFS
solutions both show the potential for multi-inch rainfall totals
from Sun onward. Ensemble probabilities suggest the potential for
two areas of particular concern, one along the central Gulf Coast
Sun-Tue, east of the mouth of the Mississippi (close to the
surface low track), and perhaps another area Wed-Thu across the
southern Appalachians/foothills, where upslope enhancement along
with deep tropical moisture inflow may enhance rainfall amounts.
PW standardized anomalies exceeding +2.5 or +3 standard deviations
are shown by the ECENS across these areas, indicating deep
tropical moisture spreading across the southern/southeastern
states. At this time, confidence is increasing into the
moderate-high realm that the Gulf Coast will see heavy rains.
Confidence is a bit lower at this time across the southern
Appalachians/foothills, and will ultimately be dependent on the
eventual track of the low pressure. Elsewhere, additional heavy
rains will be possible Sun-Mon from portions of the northern Great
Basin/northern Rockies east into the High Plains, as deep moisture
flux ahead of the upper low focuses precipitation along a
quasi-stationary frontal boundary. PWs across portions of
Wyoming/Montana east into the northern High Plains are near max
values on ESAT percentiles, with values near +3 standard
deviations possible. Thus, this appears to be another area of
potential heavy rainfall, with flooding concerns enhanced a bit
due to somewhat slow movement of the system and the persistent
surface front in the vicinity.

Additionally, temperatures are expected to be well above average
across much of the central U.S. and the Great Lakes through much
of the period. The largest max temp anomalies should occur
Sun-Mon, when high temps may reach 10 to 20 deg F above average.

Ryan


WPC medium range forecasts of 500 mb heights, surface systems,
weather grids, quantitative precipitation, and winter weather
outlook probabilities can be found at:

http://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
http://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
http://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
http://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
http://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4

$$





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