Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1128 PM EDT Sat Jul 21 2018
Valid 12Z Wed Jul 25 2018 - 12Z Sun Jul 29 2018
...Heavy Rain and Flooding for the East...
...Southwest/West heat wave into mid-late week...
...Central Rockies/Plains to Mid-MS Valley Heavy Convection...
...Guidance/Uncertainty Assessment and Weather Threats...
It remains the case that forecast predictability remains much
above normal in showing an amplified mid-uppper level trough
lingering with an axis roughly along 85W into midweek. Deep
moisture with pooled precipitable water values upwards of 2"
feeding into/ahead of the system and associated/slow moving
surface fronts/instability will prolong a pattern favorable for
cell training/repeat activity that will continue a threat of
locally heavy/excessive rainfall for the Eastern Seaboard. This
system will be surplanted by a well defined upstream trough/height
falls working across the Great Lakes/Midwest and northeastern U.S.
mid-late week whose development will focus lingering heavy
rainfall over the Northeast/coastal Mid-Atlantic and trailing to
FL/vicinity consistent with dynamic support/frontal progressions.
Flash flooding and longer term/larger scale flooding remains
possible in the wake of recent rains.
A hot mid-upper level ridge meanwhile settled over the Southwest
and up over the West along with potential for record maximum
temperatures will gradually ease through mid-later this week, but
monsoonal moisture could allow for more record high minimum
temperatures. A series of impulses rotating within and overtop the
ridge offers less predictable timings individually, but each will
provide enhanced lift. This may be especially problematic in upper
diffluent flow/boundary convergence from the central
Rockies/Plains into the mid-MS Valley where convective
complexes/thunderstorms will be fueled by inflow pooling moisture
and heavy/repeat cells may lead to a local excessive rainfall
Much of this medium range forecast remains of high confidence
during this cycle, with strong agreement and clustering among the
models and ensembles regarding these well defined and
slowly/steadily evolving features. Accordingly, the WPC medium
range forecast suite was primarily derived from a composite blend
of the latest GFS/ECMWF, GEFS and ECMWF ensemble means, the NBM,
and WPC continuity.
WPC medium range 500 mb heights, surface systems, weather grids,
quantitative precipitation, winter weather outlook probabilities
and heat indexes are found at: