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Extended Forecast Discussion
(Latest Discussion - Issued 1539Z Jun 24, 2018)
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Abbreviations and acronyms used in this product
Geographic Boundaries -  Map 1: Color  Black/White       Map 2: Color  Black/White

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
1139 AM EDT Sun Jun 24 2018

Valid 12Z Wed Jun 27 2018 - 12Z Sun Jul 01 2018

Latest guidance continues to show the mean pattern transitioning
toward an amplified western ridge/eastern trough configuration.  A
notable trend over the past day has been toward greater northward
amplitude of the eastern ridge by next weekend, with the center of
the embedded upper high possibly drifting at least as far north as
40N latitude.  This evolution would direct highest temperature
anomalies originating from the central Plains/mid-upper MS Valley
into the Northeast.  Under the base of the ridge, weather should
be unsettled across FL.  The primary uncertainty with the western
trough aloft is for its depth and the details of individual
shortwaves.  The latter will affect the forecast for what may be a
convectively active period across the Northern Tier states.

...Model discussion/preferences...
There are two issues in the forecast.  One has to do with the
longwave trough in the West and how quickly it ejects.  After
Friday, the 00z Canadian and 00z ECMWF eject it out quicker
bringing a strong low into southern Canada at a faster clip than
the other guidance.  Normally, the guidance is too quick to eject
the longwave trough and any shortwaves moving around its periphery
tend to be weak.  Since the ECMWF is quicker than its own mean,
discounted it for next weekend.

The other issue is the possibility for a baroclinically-initiated
warm core/convective low near/offshore the Southeast coast.  Most
of the guidance brings convectively-induced shortwaves out of the
southern Plains through the Mid-Mississippi Valley and Carolinas
during the short range period, causing a low to form in the
Carolinas along a weakening frontal boundary which moves offshore
by Wednesday morning.  The 00z ECMWF was quite aggressive with
this system and strengthens it to hurricane strength by next
Sunday -- likely too strong.  Since there is a building ridge
across the northern Mid-Atlantic states in all the guidance,
whatever forms offshore the Southeast would move southwest
towards/over FL sooner or later, and stuck with clustering within
the southwest side of the 00z global ensemble guidance here which
resulted in a slower westward movement than shown by
yesterday/overnight WPC continuity across FL.  At one point, 35 of
the 90 ECMWF/Canadian/GFS 00z members have the low, so it can't be
discounted.  A couple similar developments (Clara 1977 & Claudette
1985) formed as a result of convective complexes moving off the
Southeast coast; such a development is possible.

For Wednesday into Friday, a compromise of the 06z GFS, 00z ECMWF,
00z Canadian, and 00z UKMET works out fine.  Thereafter, phased
out the use of the 00z ECMWF and 00z Canadian and replaced them
with the 00z NAEFS and 00z ECMWF ensemble mean solutions.  Used
00z global ensemble member clustering to place the low offshore
the Southeast and track it into northern FL by next Sunday. 
Overall, this maintained reasonable continuity.

...Weather threats/highlights...
The system initially over the Great Lakes will likely support some
areas of moderate to heavy rainfall early in the period, most
likely from eastern Great Lakes/upper Ohio Valley into
western-northern New England.  As the upper pattern becomes more
amplified and a mean frontal boundary/low pressure persist over
the Plains and vicinity, there will be the potential for multiple
convective episodes and accompanying heavy rainfall from over or
just east of the northern Rockies into the upper MS Valley/upper
Great Lakes.  Specifics will depend on less predictable details of
shortwaves ejecting from the western trough aloft.  Currently the
best signal for highest 5-day rainfall totals exists over the
upper MS Valley/upper Great Lakes.  With lighter amounts, the
western upper trough will bring scattered rainfall to other areas
within the northern half of the region.  About 20 degrees south of
the amplifying mid-level positive height anomaly, the
Southeast/Florida will see areas of locally heavy convection
during the period, with enhancement expected from the retrograding
surface low/trough forecast to move in from offshore the Southeast

An expanding area of hot/humid conditions will likely cover a
large portion of the central-eastern U.S. by late this week and
weekend.  Currently expect the axis of highest min/max temperature
anomalies of plus 10-20F to extend from the central Plains through
the Great Lakes/Ohio Valley and into the Northeast.  Daily maximum
records appear to be more likely for warm lows than daytime highs,
though a smattering of record highs are forecast across the
northern Mid-Atlantic states and portions of New England. 
Increased low-level moisture will also lead to very high heat
index values over many areas.  Otherwise, the upper trough
amplifying into the West will bring a pronounced cooling trend. 
Locations in the northern Rockies/High Plains should see highs as
much as 10-15F below normal by Fri-Sun with less extreme anomalies


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