Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
250 AM EDT Sat Jul 27 2024

Valid 12Z Tue Jul 30 2024 - 12Z Sat Aug 03 2024

...Hazardous heat to shift from the central Plains early period to
the Northwest later next week...


...Overview...

Guidance continues to show the pattern evolving toward a
strengthening southern Rockies/Plains upper ridge which eventually
will build back across the Western U.S. by later next week. This
should help set up what should be a prolonged period of hot weather
over many areas with the most anomalous heat expected over the
Central Plains early to mid week. Shortwave energy moving through
the Midwest with eventual mean troughing that develops over the
eastern U.S., will support a broad area of showers/thunderstorms
drifting across the eastern half of the country. Despite the
overall pattern, heat increases with time across the Lower 48, with
the largest positive temperature anomalies migrating towards the
US/Canadian border by next weekend.


...Guidance/Predictability Assessment...

The guidance continues to show good overall agreement on the large
scale pattern, but plenty of uncertainty in the details. The
greatest area of uncertainty is with the timing of a shortwave
shifting out of the Northwest on Tuesday. The 18z GFS is quite a
bit faster than the other operational models as it moves across the
Upper Midwest mid week, but the new 00z run (available after the
WPC forecast was generated) trended slower. This affects the
overall shape of troughing over the Northeast later next week.
Otherwise, there is good agreement on the initial central Plains
ridge expanding Westward, but some question on timing and strength
of the ridge by next weekend over the interior West. The WPC
forecast for tonight used a non-GFS blend for the first half of the
period, increasing the ensemble mean weighting the second half to
help smooth out some of the details that will need more time to
resolve fully.


...Weather/Hazards Highlights...

The Days 4-5 Excessive Rainfall Outlooks covering Tuesday-
Wednesday night show fairly broad marginal risks across parts of
the Midwest to the Northeast where above-average precipitable water
values will be in place along weak frontal boundaries moving
through the flow. There was enough agreement in the guidance for
small slight risks across portions of the central Appalachians on
each day where flash flood guidance is typically lower and
antecedent conditions are wet. Much of the Eastern third of the
country should remain generally unsettled the rest of the week as a
upper trough is slow to budge out of the region while it generally
weakens. Elsewhere, monsoonal convection over the Southwest should
stay confined to southern areas through midweek, though it may
eventually expand a little northward depending on the shape of the
southern Rockies/Plains ridge and upper level energy rotating
around its western periphery.

The forecast pattern evolution will support an expanding area of
warmer than average temperatures across the Lower 48, possibly into
the Ohio Valley/Mid-Atlantic by next Thursday. The most persistent
and extreme high temperature anomalies should be over the Central
Plains early on where some locations should see multiple days with
highs 10-15F or so above normal and high temperatures broach 105F.
An incoming front will cause some easing of the heat late next
week. The experimental HeatRisk likewise reflects an expanding area
of Moderate to Major risks of heat- related impacts through next
Wednesday, and even some pockets in the Extreme category over the
central U.S.. Much of the West will likely see near to slightly
below normal highs through the first half of next week before the
strengthening ridge pushes temperatures above normal by next
Thursday, and more significantly so next Friday into Saturday with
lower elevations seeing 100F+ high temperatures. The Northeast
should also trend warmer with time next week.

Santorelli


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