Area Forecast Discussion
Issued by NWS Omaha/Valley, NE

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000 FXUS63 KOAX 280544 AFDOAX Area Forecast Discussion National Weather Service Omaha/Valley NE 1244 AM CDT Wed Jun 28 2017 .UPDATE...
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Issued at 1241 AM CDT Wed Jun 28 2017 Have decided rather abruptly to include counties along the Missouri River in eastern Nebraska for Severe Thunderstorm Watch 374. Apex of bow structure appears to be point toward or just south of the Omaha Metro. Thus included counties from Washington to Otoe, basically Blair to Nebraska City, along the Missouri River. UPDATE Issued at 1229 AM CDT Wed Jun 28 2017 Have added a couple of counties, namely Saunders and Lancaster, to Severe Thunderstorm Watch 374 through 2 AM. These counties appear to be in line with the strongest reflectivities and velocity signatures moving in from the west. Wind reports have been hard to come by within this line as it moved out of central Nebraska, however radar velocities are painting greater than 70kt about 3000ft off the deck. Will be monitoring evolution of the line, and may have to add counties farther east, including the Omaha Metro area.
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&& .SHORT TERM...(Tonight through Friday) Issued at 252 PM CDT Tue Jun 27 2017 Shortwave trough and mid-level speed max currently over the intermountain West, evident in water vapor loop. Expect these forcing mechanisms to move eastward the rest of the is afternoon into the evening, moving into central plains after 00UTC. CAM and Synoptic models all develop convection along attendant surface front that will be located western Dakota-Nebraska panhandle into eastern Colorado. There also is a hint of isolated convection developing along surface theta-e/thermal ridge axis that currently extends from west of Wichita to Kearney, but expect any of this activity to be diurnally driven. As mid-level forcing/ascent increases this evening, storms that develop in high plains will trudge eastward, with expected quasi-linear storm mode. CAM grow system upscale, with sustain instability of at least 2500 J/Kg, 0-6km bulk 35-50 and 0-3km bulk shear 35-50. Forecast soundings depicting mostly straight-line hodograph, and given magnitude of low-level bulk shear values, damaging winds appear the primary hazard. Weak low level backing per forecast sounding suggest potential for meso-vortex development along leading edge. Cannot discount large hail, models advect H7 temperatures 12-14C into area by 06UTC, which could mitigate large hail growth. Used CAM and NCAR Ensembles as basis for PoP, with arrival of leading edge of QLCS MCS into western parts of CWA by 02 UTC...LNK-OMA 05-06 UTC and moving into southwest Iowa 07-08 UTC. As storm moves east of primary instability axis, storms should weaken as they move into Iowa. Lingering showers and storms will exist across eastern Nebraska and western Iowa in the morning, though expect coverage to diminish due to meso-scale subsidence in wake of MCS and large scale subsidence as main short wave feature moves into Great Lakes. Reprieve will be short, as expect storms to redevelop Wednesday afternoon along fused synoptic/meso-scale boundaries left from overnight storms. Even greater SBCAPE being generated by synoptic models, with axis of 3-4.5K J/Kg extending from north central Iowa into northeast Kansas. As such will place highest PoP and storm threat across southeast Nebraska and western Iowa. Evap cooling and weak cold pool action may shove boundary a little south overnight Wednesday, though flat upper level flow will keep boundary from moving too far south. More vigorous upper level trough will dig southward from Saskatchewan, thus boundary forecast to lift back into area. Moisture pooling and relatively warm temperatures will set the stage for additional storm development, with persistent moisture advection sustaining storms into the evening and overnight. Models are generating healthy QPF, so may have to consider flood/flash flood headlines; however, given the nearly constant convective activity over the next couple days, waiting to see where boundaries and thus heavy precipitation axis will lay. Finally on Friday, there looks to be a window when storm threat will exit, as upper trough over northern plains moves east. Medium range models are differing on timing of trough movement from northern plains into Great Lakes, with GFS/GEFS slower than EC/GEM. Though there are timing differences, any lingering storms should be east of CWA by mid-day, with Friday night looking dry. Temperatures during the short term will be driven by convection, and convective debris. Wednesday looks to be a little warm in wake of MCS, with thermal ridge setting up across southern NE. Have bumped up temperatures as such. For the rest of the short term followed consensus more or less. .LONG TERM...(Friday night through Tuesday) Issued at 252 PM CDT Tue Jun 27 2017 Forecast area will remain under influence of large scale trough located over eastern U.S. through bulk of extended range. Several weak waves will ripple from northern plains across the area via northwest flow on the back side of the trough. Medium range models & ensembles are not being too bullish on precipitation chances through Sunday, with consensus models generating some slight to low chance PoPs somewhat randomly. Return flow and a more substantial short wave moves into area Sunday night into Monday, with medium range models hinting at MCS development. Exact timing and progression hard to pin down this far out. Temperatures appear season through the extended range per consensus and statistical guidance. && .AVIATION...(For the 06Z TAFS through 06Z Wednesday night) Issued at 1229 AM CDT Wed Jun 28 2017 Line of thunderstorms will move through the KLNK and KOMA TAF sites early in the TAF cycle, generally between 06Z and 10Z. Strong winds gusting over 40kt are possible, with some potential for greater than 50kt with initial surge of convection. Lingering rain and thunder is likely to hold through 12Z or so. Farther to the north, KOFK already saw the strongest storms move through that area, but several hours of rain and isolated thunder is likely there. After 12Z to 15Z time frame, most of the activity will have moved out of the area. However redevelopment is likely along an outflow boundary expected to set up near or southeast of Interstate 80. Thus KLNK and KOMA could see another round of thunderstorms between 20Z and 00Z. && .OAX WATCHES/WARNINGS/ADVISORIES... NE...None. IA...None. && $$ UPDATE...Dergan SHORT TERM...Fortin LONG TERM...Fortin AVIATION...Dergan

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