Area Forecast Discussion
Issued by NWS Des Moines, IA
Issued by NWS Des Moines, IA
Versions:
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
011 FXUS63 KDMX 221149 AFDDMX Area Forecast Discussion National Weather Service Des Moines IA 549 AM CST Fri Nov 22 2024 ...Updated for the 12z Aviation Discussion... .KEY MESSAGES... - 50% confidence for fog in the southwest half of the area tonight. Otherwise, partly to mostly cloudy elsewhere. - Above average highs this weekend. - High confidence (90%) in highs in the 30s next week. Increasing confidence (>80%) in below freezing highs following Thanksgiving. && .DISCUSSION... Issued at 408 AM CST Fri Nov 22 2024 Stratus expanded over the eastern 2/3rds of the state overnight which will keep morning lows closer to 30 degrees this morning. further west where clouds have less influence, lows in the morning will drop into the 20s. Expect the opposite warm/cold divide this afternoon as stratus will hang on in the north and east. Have lowered highs in these areas with mid to upper 30s expected. The sunnier southwest will warm into the 40s. Have raised lows for tonight since mid and upper level clouds are expected to arrive from the north around the midnight hour. It`s also probable (~40% confidence) that stratus will still have some influence in the far northeast. If clouds arrive more sparse, winds will be light enough to allow for fog to develop. This scenario appears most likely in the southwest where minor surface ridging will take place. The 06z HREF from last night has trended in this direction, with less fog coverage in the partly cloudy north, but with more widespread fog generally around the Des Moines River Valley and westward. Have decided to forego adding fog mentions in the official forecast for this issuance, but anticipate better certainty heading into today`s updates. A passing upper level system in Canada keeps the midlevel cloudiness in the north Saturday, thus stunting some of the warming over the weekend. Nonetheless, highs in the south will return to the 50s Saturday afternoon. A cloud shield surrounding a surface cyclone in the Central Plains will overspread much of the state Sunday, but the WAA regime beneath the clouds keeps the warmer temperatures coming. Perhaps a 60 degree high temperature could be recorded nearest the IA-MO border (about a 40% chance of this happening). The upper level pattern for Monday remains uncertain. The GEFS and EC ensemble camps differ on how the wave behaves over the Midwest. The GEFS prefers a slower, more intense wave and the EC settles on a more open wave. However this wave takes shape, it will energize the aforementioned surface cyclone and direct the main axis of moisture more towards the eastern half of the Corn Belt, leaving Iowa more moisture starved. Therefore, the ~10% chance for rain for the northeast seems appropriate. Depending on the synoptic forcing present, it cannot be completely ruled out to see precipitation on Monday, but it remains to be known whether or not accumulation (mostly speaking in liquid equivalent form) will be realized this day. Trends will be monitored. The 00z deterministic GFS offered a unique solution that had appreciable QPF north of here, but a quick glance at the rest of the GEFS suite quickly deemed this solution an outlier. Otherwise, CAA is a certain outcome of the passing system with highs falling back into the 30s Monday. It`s plausible (90% confidence) at this point that northern Iowa won`t see temperatures above freezing for the foreseeable future. Upper level zonal flow sets in for the midweek and offers little in the way of temperature variation. Deterministic runs still illustrate a nearby trio of waves around Thanksgiving day. Looking in ensemble land, the southern stream is becoming the favorable candidate for moisture, decreasing confidence in precipitation Thanksgiving day (current forecast is likely too generous). There`s plenty of time for that outcome to change, so stay tuned for changes to the forecast. A stout upper level closed system in Canada expands its reach southward after Thanksgiving and brings with it a shot of even colder air for the region, with most guidance keeping high temperatures for much of the state below freezing for the following weekend. && .AVIATION /12Z TAFS THROUGH 12Z SATURDAY/... Issued at 542 AM CST Fri Nov 22 2024 Patchy fog to bring occasional vsby restrictions for KMCW and KALO this morning. MVFR and IFR cigs expected to persist into the afternoon hours. Biggest change from the previous issuance was delaying the improvement onset of the cigs, keeping sites like KMCW and KALO in restrictive conditions through the duration of the TAF period. Further south and west, there remains uncertainty as to how long stratus will hang on. It`s expected for the clouds to retreat back eastward after 18z and bring VFR conditions to KDSM and KFOD by sunset. Dependent on how cloud cover behaves in the nighttime hours, fog could develop and lead to vsby restrictions after 06z. This would be most impactful for KDSM and KFOD. Have refrained from adding to any TAFs for now as uncertainty remains in how the cloud deck will behave. Trends will continue to be monitored in upcoming issuances. && .DMX WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...Jimenez AVIATION...Jimenez