Scientific Forecaster Discussion

000 fxus63 kdmx 191735 afddmx Area forecast discussion National Weather Service Des Moines Iowa 1134 am CST Tue Feb 19 2019 ..updated for 18z aviation discussion... Short term.../today through Wednesday/ issued at 431 am CST Tue Feb 19 2019 Short term... short term focus on major winter wx system to impact Iowa beginning this afternoon/evening and lasting through most of Wednesday. Synopsis... 09z Tue water vapor imagery picking up on two distinct systems... an upper low located over Montana, and another upper low located over southern Arizona. These systems phase together through this morning, as the Montana low barely propagates eastward through today, with the southern upper low riding northeastward through the positive vorticity advection side of the longwave trough, making it to the Great Lakes region by 00z Thu. Overall, models coming into very good agreement with the track, qpf, and timing of this system to where a consensus blend of short- term/hi-res models is very reasonable guidance. By 12z Wed, attendant sfc low should be in eastern KS, and by 00z Thu, sfc low into northwestern WI. -12c to -18c dry slot should be well east of the dmx cwa, with additional moisture wrapping in on the backside of this low to keep light snow/flurries going along and north of I-80 into Wednesday afternoon. By 06z Thu, dry air entrainment and large- scale subsidence win out and there should be a brief period of sunshine before the pending weekend system. Timing... noon Tuesday - afternoon/evening commute: hi-res models picking up on a small band of saturation/forcing through the dgz propagating from south to north that may yield flurries. No accumulations expected. Light snow will begin to infiltrate southwestern Iowa late afternoon. 7 PM Tuesday - 7 am wednesday: this is the time period where most snow will fall. From 7 PM Wed- 10 PM Wed, there is a band of strong low-level moisture flux convergence that is perfectly phased with a strong band of Theta-E advection that will be pushing through Iowa from south to north. Aloft, this is phased with the right entrance region of a 195 kt 250mb jet. During this narrow window, there may be more intense snowfall rates of 1 to 2 inches per hour... which could yield very poor visibility and get roads snow-covered and slippery in due haste. Around the midnight hour, the more widespread slug of moisture, Theta-E advection, moisture flux convergence, etc. Will arrive from south to north. Forecast soundings slam the low-levels, carrying saturation from the sfc well into the dgz. Using roebber snow ratios, have increased snow ratios vs guidance to the 14:1 to 16:1 range through 12z from kdsm through Kalo. Have really increased snow ratios from 12z Onward, especially in northern Iowa, as 20:1 seems very plausible. Wind speeds will be in the "slightly breezy" range of 10 to 15 mph through the event... so not quite the "snow globe" snow of the previous event, but not incredibly gusty to where blowing and drifting snow will be a significant/major issue. Wednesday am commute... snow will likely be ongoing, especially along and north of Highway 20... where an additional 2 to 4 inches may fall from 7 am through noon. Around the Des Moines Metro, and additional 1 to 2 inches of snow may fall from 7 am Wed through noon Wed. Similar to our Sunday event, flurries/light snow will likely linger through Wednesday afternoon, though very little to no accumulations are expected. Impacts... roadways will likely be snow-covered and slippery Tuesday night through Wednesday morning. This may make travel very hazardous to impossible, especially on unplowed/untreated roadways. If you must travel during this time, keep an extra flashlight, food and water in your vehicle in case of an emergency. The latest Road conditions for the state you are calling from can be obtained by calling 5 1 1. Long term.../Thursday through Monday/ issued at 431 am CST Tue Feb 19 2019 Attention shifts to yet another winter storm that may impact most to all of Iowa Saturday into Sunday. At this time, the track is still very touchy. There appears to be strong warm air advection ahead of the system, and strong cold air advection behind this system. Further, in the warm air advection Wing, long-range models show a 60kt 850mb jet that may transport moisture into this system from the Gulf of Mexico. This would certainly support thunderstorms... possibly strong thunderstorms. On the backside, the cold air advection may support decent snowfall totals. If this system pans out as is currently shown, winds will likely be very strong on the backside of this system on Sunday, which would lead to blowing and drifting of the fallen snow. At this time, too far out to get into fine mesoscale details, but the track of this potential system absolutely bears monitoring... especially for those with travel plans over the weekend. This could have significant impacts on travel across Iowa. && Aviation.../for the 18z tafs through 18z Wednesday afternoon/ issued at 1131 am CST Tue Feb 19 2019 Other than some patchy high end MVFR vsby due to fog, we will see mostly VFR conditions through 00z. Conditions deteriorate after 00z dropping to MVFR by 03z then IFR after that due to low cigs and vsbys in snow. Lowest vsbys from 3/4sm to 1/2sm expected from kfod through kdsm. && Dmx watches/warnings/advisories... Winter Storm Warning from 6 PM this evening to noon CST Wednesday for iaz033-034-044>048-058>062-072>075-084>086. Winter Storm Warning from 6 PM this evening to 9 am CST Wednesday for iaz057-070-071-081>083-092>097. Winter Storm Warning from midnight tonight to 6 PM CST Wednesday for iaz004>007-015>017-023>028-037>039-050. Winter Storm Warning from midnight tonight to noon CST Wednesday for iaz035-036-049. &&

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