Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
335 PM EDT Thu May 23 2019

Valid 12Z Sun May 26 2019 - 12Z Thu May 30 2019

...Record/Dangerous Heat for the Southeast will continue into next
week...

...Heavy rainfall possible for portions of the Plains and
Mid-Upper Mississippi Valley...


...Overview...

Through the first half of next week an amplified upper pattern
featuring a Gulf/Southeast ridge and deep western trough will
promote hot/dry conditions focused over the Southeast and at the
same time unseasonably cool temperatures from California into the
northern Plains.  A mean frontal boundary and one or more waves
between the two contrasting air masses will promote a continue
threat for heavy/excessive rainfall over parts of the Plains and
Mississippi Valley.  Although some residual western troughing
appears likely, ejection of the initial deep western trough should
ultimately lead to a less amplified pattern over the Lower 48 by
the latter half of next week.


...Guidance Evaluation and Preferences...

Today's forecast uncertainties continue to involve similar
issues--details within the initial western trough, flow heading
into East Pacific mean ridging, and Canadian troughing that may
descend far enough south to reach the northern tier of the U.S. 

Latest guidance is placing greater emphasis on the second feature
dropping into the overall western trough, a compact upper low
currently near the southern coast of mainland Alaska, while
ejecting leading energy into the Plains.  This adjustment in
shortwave details has led to two discrete surface features, a
Plains to Great Lakes wave Mon-Tue associated with the leading
ejecting energy followed by a Plains through Great Lakes/eastern
Canada wave Tue-Thu supported by the deep upper low/trough
ejecting from the West.  It remains to be seen if the first wave
will be as strong as shown by some operational models once it
reaches the Great Lakes/Northeast given a fairly flat appearance
of mean flow aloft over that region.  A model blend will help to
tone down the more extreme solutions.

Meanwhile by Tue onward multi-day trends still suggest leaning
toward maintaining some degree of eastern Pacific into western
Canada mean ridging.  Some GFS runs (including the 06Z version)
continue to be on the aggressive side in pushing height falls
toward the West Coast and the 06Z GEFS mean has trended in that
direction, but for forecasts valid on day 5 Tue most GEFS members
have trended notably higher for eastern Pacific heights compared
to two days ago.  This should allow for a lingering weak trough
aloft over at least the southern half of the West as part of a
developing split flow pattern over western North America.  At the
same time guidance has been very inconsistent and diverse for the
specifics of southern Canada troughing that could affect the
second wave lifting northeast from the Plains.  The 00Z
ECMWF/GEFS/CMC means provide the best intermediate and stable
cluster for the location and depth of the upper trough.  The 06Z
GFS and to some degree 06Z GEFS mean keep the trough farther north
than established consensus thus far.

The updated forecast starts with a blend of operational models to
reflect the emerging detail clustering for individual waves in the
Sun-Tue time frame.  Preferred guidance transitions to a blend
among the models and ECMWF/NAEFS means (the latter to capture both
00Z GEFS and CMC ensemble data) for day 6 Wed and then mostly the
means with a small lingering 00Z ECMWF component for day 7 Thu.


...Weather Highlights and Hazards...

Over the Southeast expect widespread high temperatures in the
upper 90s to low 100s F for most of the period.  Such readings
will likely challenge daily if not monthly records.  The
combination of daily highs and duration of the event may be
extreme for this early in the season.  Some locations could see at
least a 40 percent probability of seeing maximum heat indices
exceed 105F on one or more days.  Meanwhile proximity of the upper
high should limit rain chances.  Areas farther north should see
more variable temperatures depending on day-to-day frontal
position with potentially strong contrasts between the cool and
warm/hot side of the boundary.  From California into the northern
Plains the coverage of minus 10-20F anomalies for highs should be
greatest during Sun-Tue with relaxing of the upper pattern
favoring a moderating trend over this area Wed-Thu.

A broad area encompassing the north-central to south-central
Plains through the Midwest and into the western Great Lakes will
see the potential for one or more episodes of heavy rainfall with
some multi-inch totals possible for the five-day period.  Already
wet conditions will make flooding/flash flooding a significant
concern that will have to be monitored closely as specifics of
timing/intensity/duration of enhanced rainfall come into clearer
focus.  The central U.S. will also see a threat for severe
weather.  Consult Storm Prediction Center products for the latest
info regarding severe threats.

Over the West expect areas from northern-central California
through the Great Basin and northern-central Rockies to see
highest precipitation totals during the period.  Some precip could
fall as snow in highest elevations, with best potential over the
Sierra Nevada as an upper low passes overhead around Sun-Sun
night.  Uncertainty in details of flow aloft leads to low
confidence in the forecast over the Pacific Northwest late in the
period.  Current forecast preferences lean toward the lighter half
of the rainfall spread corresponding to longer maintenance of at
least some eastern Pacific mean ridging aloft. 

Rausch


Additional 3-7 Day Hazards information can be found on the WPC
medium range hazards chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

Hazards:
- Heavy rain across portions of the Plains, the Mississippi
Valley, and the Great Lakes, Sun-Wed, May 26-May 29.
- Heavy rain across portions of the Central Rockies and the
Northern/Central Plains, Mon-Tue, May 27-May 28.
- Severe weather across portions of the Central/Southern Plains,
Sun, May 26.
- Severe weather across portions of the Plains and the
Middle/Lower Mississippi Valley, Tue, May 28.
- Severe weather across portions of the Northern/Central Plains
and the Upper/Middle Mississippi Valley, Mon, May 27.
- Flooding possible across portions of the Northern/Central
Plains, the Northern Great Basin, the Upper/Middle Mississippi
Valley, the Great Lakes, and the Ohio Valley.
- Flooding occurring or imminent across portions of the Plains,
the Mississippi Valley, the Great Lakes, and the Ohio Valley.
- Flooding likely across portions of the Northern/Central Plains
and the Upper/Middle Mississippi Valley.
- Excessive heat across portions of the Southern Plains, Tue-Thu,
May 28-May 30.
- Excessive heat across portions of the Southeast, the
Mid-Atlantic, the Lower Mississippi Valley, the Southern
Appalachians, and the Tennessee Valley, Sun-Wed, May 26-May 29.
- Enhanced wildfire risk across portions of the the Southern
Rockies, the Southern Plains, and the Southwest, Mon, May 27.

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indices are at:

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