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FXUS02 KWBC 250550

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
150 AM EDT Fri May 25 2018

Valid 12Z Mon May 28 2018 - 12Z Fri Jun 01 2018

...Potential heavy rainfall event for portions of the eastern Gulf
Coast and Southeastern states late this weekend into early next

...Overview/Guidance Preferences...

The WPC forecast was based on a blend of the 12-00Z ECMWF/12Z GFS
and respective 12z ECMWF and 18z GEFS Ensemble Means Days 3-5,
switching to heavy weighting towards the 2 ensemble means days 6
Thu 31 May and Day 7 Fri 01 Jun.

The deep layer ridge building from Texas across the mid MS Valley
and weak upstream flow leads to slow forward motion of the
prospective cyclone developing in the Gulf of Mexico later this
weekend and slowly moving onshore into the Gulf coast and then
moving inland into the southeast early next week. See the National
Hurricane Center`s tropical weather outlooks and discussions for
more details on the system evolution.  The forward motion is
forecast to pick up by Wed-Thu in response to the shortwave moving
across the northern plains/Midwest.

Once this system moves towards the mid Atlantic coast next Fri,
phasing with the system over the Great Lakes could lead to
extratropical wave development, with a forming warm front moving
towards the northeast US.

A cutoff upper low initially across the Great Basin this weekend
will eject slowly northeast across the Rockies and then northern
high plains by Tue morning and crossing the northern Plains and
deamplifying as it moves into the upper MS Valley and Great Lakes
mid week.  The model agreement breaks down after the system
crosses the Lakes with lots of timing and phasing differences
regarding the southern stream system merger come Fri 01 June.

The models are in good agreement on the next mid-upper level
trough reaching the West Coast by next Tue night/Wed morning 30
May, continuing slowly inland Wed-Fri.
The 12z ECMWF falls out of phase by deamplifying the upper trough
while the 12z ECMWF and 18z GEFS ensemble means and 12z
Canadian/18z GFS were in better agreement on keeping an amplified
trough. With timing differences between the faster GFS and slower
Canadian, the preference was to use the means from the 12z ECMWF
and 18z GEFS ensembles.

...Weather/Threats Highlights...

The Gulf of Mexico low pressure will bring the potential for a
heavy rainfall event to portions of the Gulf Coast and Southeast
states. Deterministic ECMWF solutions  show the potential for
multi-inch rainfall totals from Sun onward. The two areas of
particular concern are along the Gulf Coast where the system comes
onshore Sun-Tue east of the mouth of the Mississippi and another
area Wed-Thu across the southern Appalachians/foothills, where
upslope enhancement along with deep tropical moisture inflow may
enhance rainfall amounts.

Additional moderate to locally 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 percentiles, with
values near +3 standard deviations possible. Thus, this appears to
be another area of potential heavy rainfall
near the persistent surface front in the vicinity. Convection
developing ahead of the central Plains trough leads to increased
shower and thunderstorms mid week from the central to northern

Temperatures are expected to be well above average across much of
the Plains states and upper-mid MS Valley with readings 10 to 15
deg F above average from late in the weekend through early next
week. Parts of west Texas will have multiple consecutive days over
100 degrees early to middle portions of next week. Well above
normal temperatures next Fri 01 Jun spread into the OH/TN Valleys
and lower MS Valley, with potential to reach 100 degrees in
non-coastal Texas and southern/western OK to the ARKLATEX.


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

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