Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
253 AM EDT Mon Aug 27 2018

Valid 12Z Thu Aug 30 2018 - 12Z Mon Sep 03 2018

...Weather Pattern Highlights and Hazards...

A continuing series of shortwave troughs will dig over the
northeastern Pacific to split off the west coast, but with more
energetic flow and modest precipitation potential across the
northwestern U.S...then progressing east-northeastward through
southern Canada and the U.S. northern tier. Wavy frontal systems
underneath will combine with deepened moisture and instability to
fuel periods of heavy convection, focused from the eastern
Plains/Mississippi Valley and Mid-South to the Great Lakes/Ohio
Valley/Mid-Atlantic. Meanwhile, the potential for some Pacific
energy to eventually work inland in separated southern stream flow
by next weekend may renew monsoonal connections over the Southwest
and South-central Great Basin/Rockies.

Expect a threat for locally excessive, but less organized,
thunderstorms/heavy downpours within the hot/humid sector in the
southern/eastern U.S. which may align with meso-boundaries and
local convergence effects. This potent late summer hot sector
under ridging aloft will offer lingering widespread record high
minimum temperatures into Thursday with high dew points into the
upper 60s and low 70s. A moderating front will push through the
Northeast and the mid-Atlantic thereafter, settling through the
Carolinas. The hot sector over the U.S. southern tier will
generally hold firm this week.


...Guidance and Uncertainty Assessment...

The larger scale flow pattern is reasonably well depicted in
guidance so while overall weather threats seem documented,
deterministic efforts to detail much less predictable embedded
features and local focus for convection have been suffering from
run to run variance and flip-flopping. That said, the latest GFS
and ECMWF runs have converged a bit more on their solutions over
medium range time scales, bolstering forecast confidence a bit.
Accordingly, the WPC medium range product suite was mainly derived
from a composite blend of ensembles, the National Blend of Models
and WPC continuity, with rapidly decreasing determinitsic model
input days 5-7 amid growing uncertainties.

Schichtel


WPC medium range 500 mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indexes are found 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