Area Forecast Discussion
Issued by NWS Des Moines, IA
FXUS63 KDMX 191147
Area Forecast Discussion
National Weather Service Des Moines IA
647 AM CDT Fri Aug 19 2016
...Updated Aviation Discussion for 12z TAFS...
Issued at 342 AM CDT Fri Aug 19 2016
Primary forecast concern is the potential for strong/severe storms this
afternoon/evening. Forecast Confidence = Medium
00Z upper air analysis and water vapor imagery indicated a lead upper
shortwave moving across MN with a more potent shortwave clearly
defined over southern Canada and MT. At the surface...a broad area of
low pressure was analyzed from KS newd across NE/IA and into MN/WI.
As of 09z this morning...a widespread area of thunderstorms was moving across
srn/central Minnesota associated with the aforementioned shortwave.
Further to the south over n/nw Iowa...seeing showers and weak thunderstorms
develop along the interface between a sewd moving outflow boundary
and the nose of a 30 kt swly LLJ per KOAX VWP. Expect these showers and
storms will persist for a few more hours until the LLJ weakens and
corresponding WAA/isentropic ascent subsides.
Attention then turns to thunderstorm potential in the afternoon. The upper
shortwave and corresponding H5 height falls will gradually drop swd into
the Dakotas by 00Z Saturday...again a slower progression than a few days ago.
A quasi-stationary east/west boundary is expected to set up across nrn Iowa
with a trailing cold front moving into the state from Nebraska. Expecting
at least some clearing from mid-morning until late afternoon which should allow
temperatures to warm into the 80s across central and srn Iowa. This combined
dewpoints in the 70s should yield MLCAPE values of 1500-3000 J/kg in the warm
sector ahead of the front. 00Z global and CAM solutions indicate convective
initiation along the cold front between 18-21z near the NE/IA border with a
rather quick transition to a QLCS/squall line due to linear forcing and a
deep layer shear vector oriented parallel to the front. As such...expecting
the primary severe weather threat to be strong winds...and this will likely
be maximized very close to the front where deep layer shear will range from
20-30kts favoring a multicellular storm mode. Poor 700mb-500mb lapse
rates of 5.5 C/km and low potential for persistent rotating storms should keep
the overall hail threat low. SPC currently has much of western Iowa outlooked
in a Slight Risk with a Marginal Risk elsewhere. With the slower progression
of the cold front...storms may not even reach the I-35 corridor by 7pm so this
could end up being more of a nocturnal event. Also keeping an eye on the
potential for locally heavy rainfall as PWATs pool to nearly 2 inches along the
front. Warm cloud layers will also be deep...supporting heavy downpours. The
Day 1 excessive rainfall outlook from WPC is highlighting areas west and south
of Iowa and this looks reasonable for now...so no Flash Flood headlines needed
at this point.
.LONG TERM.../Tonight through Thursday/
Issued at 342 AM CDT Fri Aug 19 2016
Main challenges in the near term of the extended. System tonight has
shown signs of slowing and have trended back timing of PoP through
the overnight and early Saturday morning hours. Overall...severe
threat appears to be earlier in the event and should wane as evening
wears on into the night. SPC`s new outlook reflects the changes as
instability maxes out at 00z and drops by about 50% by 06z. None the
less...some severe storms still likely this evening along with
locally heavy downpours with up to an inch or more of precipitation.
The main threat once again appears to be wind as instability will
quickly weaken and the bulk of the 0-6km shear again lies behind the
boundary this evening. Precipitable water is still around 1.67
inches along the boundary with sufficient warm cloud depths of 3500
to 4000m into the night. As has been advertised for several days...
the departing system will deepen as it heads into the Great Lakes.
This will help support a deformation zone extending into northern
Iowa early Saturday morning. As the storm moves east...lingering
showers will continue to occur over the northern areas of the state.
Saturday will be breezy and cooler with lessening chances for any
rain during the afternoon hours. Clouds will linger as well with
cold air advection moving southeast across the region. On Sunday
high pressure will work into the region with drier and more pleasant
conditions. Morning lows will drop into the lower 50s north and into
the mid to upper 50s over the southeast. As the high shifts east
Sunday night we will once again be looking at a return of warm air
advection aloft. Though the air remains dry...temperatures should
still recover Sunday night into Monday with highs on Monday back
into the upper 70s to lower 80s. Overall little change is anticipated
until Tuesday/Tuesday night when southwest flow aloft begins to
transport monsoonal moisture northeast from the southwest US. The
next reasonable chance of showers and thunderstorms should arrive
Wednesday into Thursday of next week though there remains some
timing differences between the Euro and the GFS with the GFS once
again faster by Day5. Will need to evaluate the timing of the
midweek system in the next few days to see how the trends shake
out. The system at that time appears to resemble a more fall like
system with some amplification as it crosses the mid Mississippi
River Valley and heads northeast. Temperatures next week should be
near to slightly below normal from Wednesday through Thursday if
the forecast rain overspreads the region as anticipated.
.AVIATION.../For the 12Z TAFS through 12Z Saturday morning/
Issued at 637 AM CDT Fri Aug 19 2016
Initial round of showers and tstms continues to exit the area to
the north and east. A lingering shower is possible at KMCW or KALO
thru 13-14z along with a brief period of MVFR cigs. Otherwise
expecting VFR conditions to prevail through late afternoon or
evening until an approaching cold front impacts the area.
Scattered thunderstorms should accompany the front although due to
timing and coverage uncertainty decided to stay with VCTS mention
until confidence increases. In the wake of the front...expect a
period of showers along with MVFR and possibly IFR cigs. Winds
should favor a southerly direction today...shifting n/nwly after