Scientific Forecaster Discussion

000 fxus63 klot 251955 afdlot Area forecast discussion National Weather Service Chicago/Romeoville, Illinois 255 PM CDT Mon Mar 25 2019 Short term... 207 PM CDT Through Tuesday night... Overall, it will be a fairly quiet period of weather through the period. Surface high pressure will settle over the area tonight into Tuesday. With clear skies and light winds, expect a chilly night tonight, with lows dropping into the 20s. Light easterly winds will continue on Tuesday with the surface high nearly overhead. However, wind speeds will likely increase a bit in the afternoon near the lake as a lake breeze push occurs. Highs on Tuesday will be a bit warmer than today, especially for inland areas where temperatures will warm into the mid to upper 40s. Colder conditions in the 30s are expected near the lake. Kjb && Long term... 255 PM CDT Wednesday through Monday... Wednesday: dry weather will continue as high pressure exits to the east. Increasing SW winds will advect in a milder air mass. Expect high in the mid 50s amidst breezy conditions and mostly sunny skies. Wednesday night through Thursday night: a deep trough over southern Canada and an associated surface low will push a cold front into the western Great Lakes by Wednesday night. The front will ultimately stall somewhere across central Illinois by Thursday night. A series of weak impulses traversing over this boundary will support sporadic chances of showers with an outside chance of an isolated embedded thunderstorm. Overall, most of the period will be dry across the area, though the highest chances of showers will be later Thursday night. Friday through saturday: focus then shifts to the potential for a couple rounds of potentially moderate to heavy rain across the County Warning Area. The focus for heavier rain will be dependent on the location of the stalled surface/low-level boundary. This boundary should remain across the southern County Warning Area or south of the County Warning Area Friday before shifting northward into the central County Warning Area as the surface low tracks NE across the area Friday night into Saturday. Heavy rain potential will be enhanced by a few factors: 1) high precipitable water anomalies for late March of around two Standard deviations and averaging near the climatological maximum values based off sounding climatology at dvn and ilx. 2) stalled frontal boundary draped west to east across the County Warning Area. 3) notable low-level moisture transport in response to increasing SW flow ahead of low pressure in the central Great Plains, particularly Friday afternoon and night. Given the recent river flooding across the northwest half of the cwa, there is increased concern of heavy rainfall occurring across those river basins. Long-range guidance, including forecast ensembles, continue to shift the heavy rain axis north and south from run to run. However, it is becoming more evident that the axis of heavy rain will affect at least portions of the cwa, with the period of heaviest rain with possible a few embedded thunderstorms occurring Friday afternoon into Saturday morning. Consensus forecasts produce widespread 1" to locally 2"+ within the County Warning Area. Continue to monitor the latest forecasts and statements for the latest details. Saturday night through monday: high pressure will settle behind the late-week system, resulting in dry conditions into early next week. Unseasonably cold conditions can also be expected during this time, with highs in the upper 30s to mid 40s on Sunday, coldest near Lake Michigan. Kluber && Hydrology... A front will stall over the region late in the week, which will result in a couple bouts of moderate to heavy rainfall from Thursday night through Saturday. Isolated thunderstorms are also possible. Significant rainfall totals of one inch to in excess of two inches are possible. Despite it being about five days from the start of this potential event, model guidance is in very good agreement on area wide rainfall totals of at least one inch. Given that the air mass over the region will be unseasonably moist and supportive of efficient rainfall rates, pattern recognition of past similar heavy rainfall setups suggest that event totals of 2.5" or higher will be possible in spots. It's too early to be able to pinpoint specifics for which areas will receive the highest rainfall totals, but even the ensemble baseline of 1"+ is concerning for renewed sharp river and stream rises and lowland flooding. This includes the most impacted basins the past few weeks, the rock, Pecatonica and Fox, with moderate flooding still ongoing along portions of these rivers. Soil moisture remains above normal for late March, so increased runoff will be a concern with respect to river flooding area wide. An esf/hydrologic outlook has been issued to highlight the concerns for this potentially significant rainfall event. Castro && Aviation... for the 18z tafs... Quiet weather is expected through the period. Northeasterly winds today will gradually become less gusty through the afternoon, then weaken to around 5 kt tonight into Tuesday morning as a surface high shifts over the area. Expect easterly winds to continue on Tuesday, with a bit of an uptick in speeds closer to 10 kt in the afternoon as a lake breeze push occurs. Otherwise, expect VFR conditions. Kjb && Lot watches/warnings/advisories... Illinois...none. In...none. Lm...Small Craft Advisory...nearshore waters until 4 am Tuesday. && $$ Visit US at http://weather.Gov/Chicago (all lowercase)

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