Area Forecast Discussion
Issued by NWS Quad Cities, IA IL

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678
FXUS63 KDVN 180553
AFDDVN

Area Forecast Discussion
National Weather Service Quad Cities IA IL
1253 AM CDT Sun Aug 18 2019

...AVIATION UPDATE...

.UPDATE...
Issued at 950 PM CDT Sat Aug 17 2019

Classic bow echo with rear inflow jet west of Sioux Falls, SD
moving southeast at 50 mph. Damaging winds of 70 mph have been
reported. This was ahead of a cold front that extended from
northern MN into eastern SD and nw NE. A warm front was draped
from southern MN to southern WI. The edge of a strong cap was
across northern NE. Plenty of CAPE was evident in NE/western IA
with values of 4000+ J/kg with bulk shear 40+ kt. Strengthening
LLJ extended from the central Plains into nw IA. All signs point
to this bow echo to move southeast along the CAPE gradient as the
LLJ veers into eastern IA late tonight.

The latest runs of the HRRR/NAMNEST suggest a severe squall line
moving rapidly southeast across the DVN cwa in the 2 am to 8 am
time frame. Damaging winds of 60 to 70 mph are possible and can`t
rule out an isolated spin up tornado with decent shear and
sufficient CAPE even in the early morning hours.

&&

.SYNOPSIS...
Issued at 307 PM CDT Sat Aug 17 2019

A surface trough sits along the Iowa and Illinois border and then
curves west along the Iowa and Illinois border. Skies were partly
sunny this afternoon. Temperatures ranged from 77 degrees at Dubuque
and Cedar Rapids to 84 degrees at Muscatine and Mount Pleasant.
Dewpoints ranged from the mid 60s north of a Cedar Rapids to
Sterling Rock Falls line to around 70 south of a Fairfield to
Galesburg line.

&&

.SHORT TERM...(This Evening through Sunday)
ISSUED AT 307 PM CDT Sat Aug 17 2019

Models are in good overall agreement.  The main forecast
concerns are the threat for showers and thunderstorms tonight
into Sunday.

In the near term, expect showers and thunderstorms to continue
to develop in this area of convergence across the area but that
will shift to the north and east late this afternoon and a quiet
period is expected later this evening.

The low level jet is forecast to increase this evening with the
 nose of the level jet spreading northward from northern Missouri
 at 00 UTC to near the Highway 20 corridor by 6 to 9 UTC. Strong
warm air and moisture advection with the low level jet will increase
CAPE across the area this evening with widespread showers and
thunderstorms developing. The main threat area looks to be north
of Interstate 80 overnight as storm developing south shift
northward on the nose of the strengthening low level jet and weak
synoptic lift overspreads the area.

0 to 6 km shear is increasing to 30 to 40 knots as the low level
 jet increases with most unstable CAPE of 1000 to 2000 J/KG.
Models are also predicting 0 to 1 KM shear around 20 knots with
SRH around 100 to 200 m2/s2. The main threat from these storms
will be large hail, damaging winds, and mesovortex tornadoes
along the squall line.

Precipitable water will increase ovenight to 1.75 to almost 2.0
inches so heavy rainfall will also be possible. Some locations
ail see 1 to 2 inches of rainfall but if storms move over the same
area repeatedly, rainfall amounts over 3 inches are also
possible. This may lead to localized flash flooding but not a
widespread flash flooding event.

Convection will quickly come to an end between 12 to 15 UTC with a
break before a cold front approaches the area tomorrow afternoon
ahead of a trough moving from the Northern Plains into the Upper
Great Lakes. A marginal risk of severe thunderstorms exists east
of a line from Ottumwa to Freeport.

.LONG TERM...(Sunday Night through Saturday)
ISSUED AT 307 PM CDT Sat Aug 17 2019

A brief period of heat and humidity early in the week then
temperatures at or slightly below normal.

Sunday night and Monday
Assessment...low to medium confidence

The models push a cold front into the area Sunday night before it
stalls. Where it stalls will dictate how cool it gets behind the
front. Some solutions stall it out over the area while other push it
all way through the area.

Right now it appears that it will stall out across the far east and
southern areas. Forcing along and ahead of the front will act on the
sufficient moisture to generate convection that will end from
northwest to southeast Sunday night and Monday morning.

Monday night
Assessment...medium confidence

High pressure moving through the Midwest should bring quiet and dry
conditions to the area.

Tuesday
Assessment...low to medium confidence

The models push another cold front with an associated upper level
disturbance through the area. If sufficient moisture is available
scattered showers and storms will develop along the front. The model
consensus has chance pops for the area.

The timing of the front and storms will play into how warm it gets
Tuesday. If cloud cover is extensive enough then temperatures may be
cooler than forecast. Regardless it will be humid with heat index
readings at least in the 90s.

Tuesday night on...

Tuesday night and Wednesday
Assessment...low to medium confidence

The models vary on how quickly the front finally exits the area but
it appears it will exit the area Wednesday afternoon. Right now the
model consensus has chance pops Tuesday night and slight chance to
chance pops on Wednesday.

Wednesday night through Thursday night
Assessment...medium to high confidence

The model consensus has high pressure moving through the Midwest
resulting in quiet and dry conditions.

Friday through Saturday
Assessment...medium confidence

High pressure moves into the eastern Great Lakes with return flow
developing across the area. The models diverge with their respective
solutions in regards to what happens and it appears to be tied to
available moisture. The model consensus has mainly dry conditions
during the daylight hours with slight chance pops Friday night as
an upper level disturbance moves through the area.

If sufficient moisture is available I cannot rule out the
possibility of afternoon convection developing during peak heating
each day.

&&

.AVIATION...(For the 06Z TAFS through 06Z Sunday Night)
ISSUED AT 1253 AM CDT Sun Aug 18 2019

Potentially several rounds of showers and thunderstorms are
possible at all TAFs sites overnight into Sunday morning. MVFR
ceilings and visibilities are expected with these showers and
storms with periods of IFR conditions. Heavy rain and strong
winds will be possible with the strongest storms. Conditions are
forecast to improve to VFR conds on Sunday. A cold front will
arrive Sunday evening with winds becoming northwest with the
passage of the front.

&&

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

$$

UPDATE...Haase
SYNOPSIS...Cousins
SHORT TERM...Cousins
LONG TERM...08
AVIATION...Haase



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