368
FXUS02 KWBC 190700
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
300 AM EDT Wed Jun 19 2024

Valid 12Z Sat Jun 22 2024 - 12Z Wed Jun 26 2024

...Record-breaking heat from the Midwest to the East Coast expected
to moderate early next week, while some heat shifts into the
South/Plains...


...Heavy rainfall threat from the Upper Great Lakes into northern
New England Saturday-Sunday...


...Overview...

Guidance still shows the upper pattern settling into a more typical
early summer regime with upper ridging settling across the far
southern tier while a leading northern tier upper trough reaches
the East by the start of next week and upstream Pacific energy
progresses along/north of the Canadian border. These northern
features will support the two primary frontal systems of interest
during the period. The eastern system will bring a heavy rainfall
threat from the Upper Great Lakes into New England during the
weekend and by early next week will help to moderate the area of
hazardous heat extending from the Midwest into the Mid-Atlantic
through Sunday. However heights aloft will tend to remain somewhat
above normal otherwise, ultimately leading to fairly broad coverage
of above normal temperatures over the lower 48. To the south of
the southern tier upper ridge, Potential Tropical Cyclone One (see
NHC for updates) should dissipate over Mexico before the start of
the forecast period while NHC is monitoring potential for another
western Gulf feature that could bring additional rainfall into far
southern Texas.


...Guidance/Predictability Assessment...

Dynamical and ECMWF-initialized machine learning (ML) guidance
agrees in principle on the overall forecast pattern, but there are
important detail differences yet to be resolved. On average the
guidance has recently trended a little more open with the upper
trough supporting the system tracking east/northeast from the
northern Plains Saturday onward. A mean of ML solutions would
continue to support an average of operational guidance, though
notably the MLs have not favored the deep side of the guidance per
some recent ECMWF runs. Meanwhile latest trends have gone in the
way of some earlier GFS runs upstream, with a greater signal toward
a more sheared presentation of incoming Pacific energy as it
continues across the northern tier U.S. and southern Canada. This
leads to a faster trend for the Canadian surface low and trailing
front into the CONUS. By next Wednesday the 12Z ECMWF seems to
stray on the fast side with bringing in the next larger scale
feature (some combination of lingering Northeast Pacific upper low
and upstream energy). Operational guidance has trended in the way
of prior MLs in suggesting whatever energy/surface low pressure
there could be over the Gulf would be more suppressed, given a
stronger southern tier upper ridge.

12Z/18Z guidance comparisons led to starting the forecast with an
operational model composite early in the period, followed by some
incorporation of the 18Z GEFS and 12Z ECens/CMCens means later in
the period as details become somewhat more uncertain.


...Weather/Hazards Highlights...

Developing low pressure tracking from the northern Plains/Upper
Midwest toward New England/southern Canada along with its
associated fronts will interact with moisture that has been over
the Upper Midwest for a number of days to support an area of heavy
rainfall potential during the weekend. The Day 4 Excessive Rainfall
Outlook for Saturday made modest adjustments to the existing Slight
Risk area covering northern parts of the Great Lakes/Midwest based
on latest guidance. The surrounding Marginal Risk area remains
fairly close to continuity over the Midwest through central Great
Lakes, but parts of the Northeast has been added based on recent
faster trends for potentially significant rainfall. By Day 5,
uncertainty over the details of surface low pressure complicate the
forecast somewhat. Currently the best signal for heaviest rainfall
exists over northern New England so a proposed Slight Risk area
covers this region of relatively higher probabilities. A Marginal
Risk area extends southwestward into the central Appalachians,
incorporating guidance signals for some locally heavy rainfall and
typical terrain sensitivity.

Elsewhere, guidance is not unanimous on the magnitude but in
general is suggesting the flow of enhanced moisture into far
southern Texas to the north of another possible area of western
Gulf low pressure during the weekend. Thus the Days 4-5 EROs have
introduced a Marginal Risk area, given QPFs in some models and wet
antecedent conditions. Meanwhile some scattered diurnal convection
may persist over parts of the Four Corners states. Anomalously
high precipitable water values suggest some activity could be
locally heavy but so far guidance has not been quite organized
enough to merit a risk area. Parts of the Southeast/Florida may
also see locally heavy rainfall but again without enough
organization to favor a risk area. During the early-middle part of
next week the next frontal system reaching the Plains/Mississippi
Valley may produce showers and thunderstorms of varying intensity.

Locations from the Midwest into Mid-Atlantic should continue to see
temperatures 10-15F above normal through the weekend. These
anomalies would translate to highs well into the 90s, along with
lows in the upper 60s to upper 70s providing little overnight heat
relief. Daily records for max/warm min temperatures will be
possible within the above areas through the weekend, up through
warm minimum temperatures early Monday (if they hold on through the
calendar day). The front progressing into the East will suppress
the heat southward Monday-Tuesday. The warm sector ahead of the
next frontal system will spread very warm to hot temperatures into
the West during the weekend and continuing into the Plains next
week, while readings remain somewhat above normal back through
California into midweek. By Sunday-Wednesday expect highs well
into the 90s and high dewpoints over the South, plus some 100F+
readings over the central/south-central Plains, leading to
increasing coverage of maximum heat index values in the 105-110F
range across these regions (with HeatRisk in the major to locally
extreme category).

Rausch


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

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation forecast (QPF), excessive rainfall
outlook (ERO), winter weather outlook (WWO) probabilities, heat
indices, and Key Messages can be accessed from:

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/#page=ero
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml
https://www.wpc.ncep.noaa.gov/#page=ovw






$$