Prognostic Meteorological Discussion
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FXUS02 KWBC 270635
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
234 AM EDT Sun May 27 2018

Valid 12Z Wed May 30 2018 - 12Z Sun Jun 03 2018

...The decaying remains of what is currently Subtropical Storm
Alberto should bring unsettled conditions to the Great Lakes and
Ohio Valley by mid-week...


...Synoptic Overview...

At the onset of the forecast period, May 30th, Alberto should be a
tropical depression based on the latest National Hurricane Center
forecast with a location over the lower Ohio Valley. It should be
in the process of getting absorbed by a mid-latitude shortwave
with a gradual eastward adjustment the following couple of days.
Without significant steering flows in its vicinity, the upper
reflection of this system could linger over the Eastern Seaboard
into next weekend. Elsewhere, the pattern is a bit less clear
given a series of uncertainties which prevail offshore over the
Pacific. On Thursday/Friday, an upper trough is forecast to dig
into the western U.S. with the southern extent of height falls
crossing northern Arizona by early Friday. This system should
carry its influences into the Great Plains over the weekend while
another upper low looms offshore of Vancouver Island. The state of
each of these mentioned systems is somewhat in flux into next
weekend given large-scale model differences. It does appear safe
to say some form of negative height anomaly will linger off the
British Columbia coast into next Sunday, June 3rd.


...Model Guidance/Preferences...

The latest 00Z model suite has shown a slight northward adjustment
in the position of Alberto as indicated by the GFS/UKMET
solutions. Will see if the 09Z advisory makes any major
adjustments to the medium range forecast track. Regardless, it
appears the remnant circulation will eventually become absorbed by
an upstream mid-latitude trough in the vicinity of the Great Lakes
by Thursday. At this point, there is general model agreement in
dragging a broad mid-level weakness toward the Eastern Seaboard by
next weekend. Given significant distance from the stronger belt of
westerlies across eastern Canada, the system could continue to
linger while gradually shearing.

Elsewhere, a modest shortwave will be in the process of moving
through the Dakotas and Upper Midwest on Wednesday. Solutions
continue to vary with surface strength as the 12Z ECMWF remained
the weakest, possibly on the order of 6 to 10 mb. Recent GFS
forecasts favor keeping this cyclone center very near the remains
of Alberto on Thursday while other global models lift the upstream
circulation well up into western Ontario. Gradually the
synoptic-scale focus will shift westward to amplified flow setting
up over the southwestern U.S. on Thursday/Friday. Ensemble
spaghetti plots had been quite noisy but the 12Z cycle offers a
bit more certainty in the pattern through Thursday or so. Like
preceding shortwaves and closed lows crossing this region in
recent times, most have lifted sharply north-northeastward with
this system likely following suit. Uncertainty grows from Friday
onward with what state this feature will be in. By Saturday
morning, scenarios are variable as indicated by the notable
ensemble spread. Considering recent trends, the 00Z GFS/CMC have
moved in opposite directions with the latter significantly slowing
while the 12Z ECMWF sits well south and east of other global
models.

As this system wobbles about the middle of the country,
uncertainty across the Pacific looms over the forecast. Run-to-run
model continuity is rather poor at best as they cannot seem to
agree on whether a trough or a ridge will sit over Vancouver
Island early next weekend. Ensemble means seem to favor a
flattening trough which would be a nice compromise from the myriad
of solutions at hand.

Took a multi-operational model consensus through Day 4/Thursday,
utilizing a combination of the 12Z GFS/ECMWF/CMC/UKMET. From Day
5/Friday onward, favored a solution along the lines of the 12Z
GFS/ECMWF and 12Z GEFS/ECMWF/NAEFS ensemble means with more
emphasis on ensemble solutions deeper into the forecast.
Confidence becomes slightly below average next weekend given the
messy western North America pattern.


...Weather/Threats Highlights...

It should be a rather wet period for much of the Ohio Valley and
Great Lakes region as Alberto pushes through the region on
Wednesday/Thursday. Abundant tropical moisture spilling into the
southern and eastern states will also raise rainfall chances
through much of the week. In these latter locations, it will
likely be more difficult to pinpoint where isolated heavier
amounts will occur given unknown forcing mechanisms and placement
of mesoscale features. Farther upstream, highly diffulent flow in
advance of the upper low across the central Great Basin will
foster the development of heavy rainfall over the Northern Rockies
into the Dakotas. The 00Z GFS/12Z ECMWF show 24-hour totals
possibly in the 2 to 4 inch range given slow progression of
precipitation elements. This signature slowly drags eastward into
the Upper Midwest by the weekend.

Excessive heat will dominate the forecast throughout much of next
week. While locations over the West Coast can expect readings near
or slightly below climatology due to the deep upper trough, this
will truly be the anomaly in the pattern. Widespread temperature
records are likely from the Great Plains eastward, particularly
for the warm overnight minima. The hot spot should be the Southern
Plains where numbers may exceed 105 degrees along the Rio Grande
while century degree readings also encompass much the region
surrounding the Red River. A true heat wave is in store as several
days of such conditions are likely. Even surrounding locations
will get in on the action as widespread 90s are expected across
the Central Plains as well as the lower Mississippi Valley.


Rubin-Oster


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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