Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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FXUS63 KDVN 250521
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1221 AM CDT Fri May 25 2018

...AVIATION UPDATE...

.SYNOPSIS...
Issued at 323 PM CDT Thu May 24 2018

Mostly sunny skies were across the area with mid afternoon
temperatures in the 80s to around 90, with dewpoints mainly in the
lower 60s.

&&

.SHORT TERM...(This Evening through Friday)
ISSUED AT 323 PM CDT Thu May 24 2018

Forecast focus on a chance of thunderstorms Friday.

Tonight: Dry much of the night then a small chance of showers
or storms towards morning in our far western counties as a short
wave trough approaches from the west. For now I will maintain
chance pops as models suggest coverage to be isolated or
scattered. Should be a warmer night with a continued southerly
wind. Lows will be in the mostly in the mid 60s.

Friday: SPC has a slight risk of severe storms late in the
afternoon and into the evening in our nw cwa. Prior to that there
will be a continued small chance for showers or storms. Deep
layer shear will be weak but SBCAPES are expected to climb to
3000-3500 J/KG, more than sufficient for multi-cell storms
producing large hail and damaging wind. PWAT`s should push above
1.50 inches which will also support locally heavy rainfall in the
storms. The main question remains how high will surface dewpoints
get as some models suggest lower 70s, while others keep the
dewpoints in the mid to upper 60s. The initiation of storms is in
question also as the GFS has the storms firing in se MN towards
evening but the NAM has them developing in northeast IA. Anyway,
this will be a very warm and more humid day with highs in the
upper 80s to around 90, with heat index values in the lower 90s.
While this is no where near advisory criteria you should still
take precautions as this will be the first summer-like day.

.LONG TERM...(Friday Night through Thursday)
ISSUED AT 323 PM CDT Thu May 24 2018

An extended stretch of summer-like weather will dominate the
Memorial Day holiday weekend well into next week. The biggest
forecast challenge is the potential for severe weather Friday
evening.

An upper level trough moving through the upper Mississippi River Valley
region, combined with thetae advection, will likely trigger at least
scattered thunderstorms over northeast IA/southwest WI/SE MN area by
late afternoon. Based on the timing of separate weaker vort maxes in
the flow, these could arrive in two separate waves, with one underway
over eastern IA into NW Illinois early in the evening and a
separate wave a few hours later.

As eluded to in the short term discussion, the NAM, GFS and
several convective models, based on an assumption of surface
dewpoints reaching the lower 70s, would place eastern IA on the
eastern edge of a SBCAPE gradient with 3000 plus J/KG to the west.
This leads to the development of an outflow driven, MCS with a
damaging wind threat that would race south-southeast across
eastern IA and northwest IL in the evening, in an overall
weakening fashion. Considering the airmass source region, our
relatively dry surface conditions and immature crops, these model
depicted surface dewpoints look on the high side. The slightly
drier, more realistic ECMWF dewpoint values in the 60s would limit
SBCAPES closer to a 1000 to 2000 J/KG range, which combined with
weak wind shear, would point toward a more limited severe weather
threat.

SPC has a slight risk over our northwest, and marginal risk over
the remainder of the forecast area, based on the more unstable
solutions. For now, forecast confidence is low that there will be
severe weather and chance pops area maintained for mainly evening
thunderstorms. Besides the damaging wind and lesser hail threat,
heavy rainfall will be possible with any storm due to what looks
to be slow movement of storms early on and the high precipitable
water.

Saturday through next week: The unseasonably warm and somewhat humid
conditions will be the main story for the holiday weekend with a low
potential for storms mainly in the south Saturday. Storm chances
through the rest of the weekend look very low as the upper level
ridge rebuilds over the region. Highs look to reach the upper 80s to
around 90 each day with overnight lows limited to the 60s as
dewpoint temperatures hold in the 60s. Resulting heat index values
reach the lower 90s, which will not be particularly high or
hazardous, based on mid summer standards. None-the-less, it will be
uncomfortable and unexpected for some given that it is still May.

By the middle of next week, temperatures should cool some, with
increasing rain chances as the flow becomes more progressive.

&&

.AVIATION...(For the 06Z TAFS through 06Z Friday Night)
ISSUED AT 1217 AM CDT Fri May 25 2018

A warm air mass is over the Midwest early today, with a few areas
of storms in the region. These are far and few between, and
predicting when and if any will impact our local area will be very
challenging beyond the initial 6 hour period. While models are
certain that a cluster of storms will race northeast from Missouri
early this morning, that cluster on radar and satellite, is
dissipating. Thus, we`ll watch trends, but will pull back on
the thunderstorms from the earlier issuance and wait to see if
any convection can refire and move northeast. As of 1220 AM, there
isn`t any. Outside of storms, the weather is VFR with light south
to southwest winds through Friday night.

&&

.DVN WATCHES/WARNINGS/ADVISORIES...
IA...NONE.
IL...NONE.
MO...NONE.
&&

$$

SYNOPSIS...Haase
SHORT TERM...Haase
LONG TERM...Sheets
AVIATION...Ervin



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