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Displaying AFOS PIL: AFDDMX Received: 2018-07-12 17:24 UTC

FXUS63 KDMX 121724

Area Forecast Discussion
National Weather Service Des Moines IA
1224 PM CDT Thu Jul 12 2018

.SHORT TERM.../Today through Tonight/
Issued at 339 AM CDT Thu Jul 12 2018

Main concerns today center around temperatures and precip chances
late. Have an area of high pressure that is exiting the area 
centered off to the east across the Ohio Valley/eastern Great 
Lakes region. An area of low pressure is centered over 
central/northern Manitoba with a sfc trough axis stretching 
southward into the eastern Dakotas/Minnesota, then southwestward 
into NE Colorado. This area of low pressure still move eastward 
into the Hudson Bay region with an associated cold front expected 
to approach the state by late today into tonight. Return flow 
continues across the state with moisture advection and WAA across 
the forecast area ahead of the southward moving cold front. This 
sets up conditions for another hot and humid day across central 
Iowa. Highs expected to top out in the low to mid 90s as H85 temps
climb into the low to mid 20s Celsius with dewpoints in the low 
to mid 70s across much of the area. This means that maximum heat 
index values this afternoon will increase to around 100 to 105 
degrees with a few sites possibly higher. Therefore will continue 
the heat advisory for the area. 

As the cold front approaches northern Iowa, thunderstorms are 
expected to develop along and ahead of the front across South Dakota 
and southern MN by late afternoon/early evening. Some potential that 
ongoing activity across MN could end up pushing the boundary a bit 
further south with some chances for storms in the far north toward 
early evening. Therefore did include some small pops through the 
evening hours. However the more likelihood will be for activity to 
spread into northern Iowa by late evening and into the late night 
hours as the frontal boundary continues to work its way south into 
northern Iowa toward early Friday. Main moisture transport situated 
out to the west of the state by later today, so this should also 
limit chances for activity in northern Iowa toward late afternoon. 
Moisture transport better into the state toward late 
evening/overnight, thus have the highest pops in the north during 
that time. Also with the front approaching the northern forecast 
area, did raise min temps tonight as moisture likely to pool some 
ahead of the boundary and some cloud cover around, thus temps likely 
to remain mild overnight. Therefore went with lows across the area 
in the low to mid 70s.

.LONG TERM.../Friday through Wednesday/
Issued at 339 AM CDT Thu Jul 12 2018

As upper level short wave trough lifts into Hudson Bay, trailing 
surface boundary will drift southward into the state. A broken 
line of showers and storms will be primarily over northwest into 
far north central Iowa by 12z Friday perhaps reaching down toward 
the Highway 20 corridor by midday. 00z HREF convective allowing 
models (CAMs) along with many of the 00z global models agree the 
location of rainfall, though a notable outlier is the GFS. The GFS
over the past several days has been farther south with its rain 
and been the wetter model compared to the CMC and ECMWF. Still, 
expect scattered showers and storms to spread south and east 
through the day into Friday night as the boundary sinks farther 
into the state. While there will be a moderate amount of 
instability by later Friday afternoon, the severe threat will be 
offset by overall weak deep layer shear. There still could be some
severe wind gusts as delta theta-e differences reach 30K to 
perhaps 40K in both the NAM and GFS with model soundings also 
indicating around 1000J/kg of downdraft CAPE. Overall, the 
concern will be on the possibility of locally heavy rainfall, 
especially Friday night into Saturday morning. Precipitable water 
values will range from 1.5 to around 2 inches during the day and 
ramp up to between 2 to 2.5 inches in a narrow northeast-southwest
oriented band across central Iowa Friday night into the early 
hours of Saturday. Mean flow of 15 to 20 knots will be common with
deep warm cloud depths indicating efficient rain processes still 
possible. Blended in WPC with initial Superblend guidance to 
arrive at 12z Friday through 12z Saturday rain total of half an 
inch to around 1.5 inches over central and northern Iowa. Rain 
will stay in the forecast through Sunday as the boundary becomes 
more diffuse over the region. The best rain/storm chances will 
shift into eastern and southeastern Iowa through the day on 
Saturday. Overall, locally heavy rainfall will still be possible 
more so over the southern half of the forecast area on Saturday as
the heavy rain parameters remain similar. Additional discussion 
on locally heavy rainfall leading to flash flooding and/or river 
flooding can be found in the Hydrology discussion below. 

Rain chances will lower across central Iowa Saturday night into 
the first part of Sunday. Upstream, the next shortwave trough will
be move across the northern part of the central Canadian 
provinces. This will bring a cold front sweeping through the state
later Sunday into early Monday. Drier and cooler weather with 
highs generally in the 80s will follow behind the front as surface
high pressure moves into the region to start the week. The upper 
level pattern will transition next week as well as the trough digs
over eastern North America while the ridge builds over the 
western US. This will place Iowa in northwest flow with a 
shortwave trough riding in this flow bringing scattered showers or
storms back into the forecast by later Tuesday if not more likely


.AVIATION.../For the 18Z TAFS through 18Z Friday afternoon/
Issued at 1222 PM CDT Thu Jul 12 2018

VFR conditions will persist through much of the period. Primary
hazards through the period is the potential for thunderstorms
across northern Iowa late tonight into Friday. 


Issued at 339 AM CDT Thu Jul 12 2018

Over the last week, there has been some rain over northern Iowa with 
amounts ranging from a general quarter to a half an inch to 
isolated areas having over an inch. For southern Iowa, it has been
over a week since any appreciable rain has fallen and drought 
conditions continue over far southern areas. Despite these drier 
recent conditions, models and WPC guidance are pointing to 
rainfall amounts ranging from a general 1 to 2 inches over central
and northern Iowa. The NAM and GFS are also beginning to show the
upper end potential that may be possible with the NAM having a 
swath of 4 to 6 inches along the Highway 20 corridor with the GFS 
having an area of 4 to 5 inches over northern Iowa. While there 
is still uncertainty in the placement of the higher rain amounts 
with WPC and NAM favoring more central Iowa compared to the 
remainder of the global models favoring northern Iowa, rain 
amounts of this magnitude would lend themselves to the possibility 
of river flooding and flash flooding. WPC has a slight risk of 
excessive rainfall exceeding flash flood guidance over portions of
Iowa each day Friday and Saturday. As for river flooding, concern
is greatest for the Des Moines River basin. There is still 
flooding along the upper reaches of the West Fork with elevated 
levels farther downstream, including the Saylorville Resevoir from
storing previous rainfall in June.


Heat Advisory until 8 PM CDT this evening for IAZ004-005-015-023-

Heat Advisory until 8 PM CDT this evening for IAZ006-007-016-017-



SHORT TERM...Beerends
LONG TERM...Ansorge