Area Forecast Discussion
Issued by NWS North Platte, NE
Issued by NWS North Platte, NE
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675 FXUS63 KLBF 052329 AFDLBF Area Forecast Discussion National Weather Service North Platte NE 629 PM CDT Fri Jul 5 2024 .KEY MESSAGES... - A couple waves of thunderstorms are possible Saturday through early Sunday with strong to severe activity possible for much of Saturday. - More widespread rain and thunderstorms will accompany a passing cool front Sunday, which will bring to threat of locally heavy rain in addition to holding afternoon temperatures in the 70s. - Overall weather pattern next week will be dominated by strong upstream ridging and highly amplified northwesterly flow locally. This suggests fairly dry weather and moderating temperatures. && .SHORT TERM /THROUGH SUNDAY NIGHT/... Issued at 221 PM CDT Fri Jul 5 2024 Early morning satellite analysis shows a modest disturbance tracking across southwest Nebraska. This is producing enough lift via modest h7 convergence within a weakly unstable environment to drive isolated showers. Further satellite analysis depicts some convective elements to this though lightning has not recently been observed with this activity. On a broader scope...large scale ridging remains anchored across the West Coast with departing trough diving east- southeast across the Great Lakes. Slight northwesterly flow remains in place across much of the central and northern Plains and will dominate the upper-level pattern through the valid forecast period. Aforementioned mid-level wave producing light showers across the southwest will continue to shift east-southeast through early afternoon. Have maintained Chance (< 35%) PoPs across our southern three counties with anticipation that some strengthening may occur as temperatures climb and instability builds. Even so, MUCAPE values peak around 500 j/kg in the area within weak shear (< 30 knots) so no severe weather is expected. This may allow more precipitation to reach the ground, however, as a few stronger updrafts may develop with spotty QPF amounts indicated by various hi-res guidance. Behind this, northwesterly low-level flow continues to reinforce a fairly dry airmass in place. Precipitable water values across much of Nebraska remain below normal with latest NAM guidance suggesting values nearing 50% of normal. With steady northwest winds, mechanical mixing should support gusty winds and deeply mixed boundary layers. Signs of this are already showing up as of 17z with an expansive cu field across the Sandhills. Can`t rule out a widely isolated shower or weak thunderstorm develop this afternoon and will cover this potential with a 15% PoP (Slight Chance). Friday morning HREF guidance shows this potential with fairly aggressive QPF painting across the area. While this is likely overdone compared to what will actually occur, it speaks to the potential of development that may occur anywhere in short order. Activity, should any form, will likely dissipate with loss of daytime heating this evening. Saturday...Winds quickly veer this evening as weak low pressure develops over southeast Wyoming in response to approaching Alberta Clipper. Timing of this feature makes for an interesting forecast as greatest upper-level dynamics including associated height falls and DCVA favor the morning hours Saturday. With the main surface low progged to slide west of the area, southerly flow ahead of this will draw in theta-e rich air with increasing instability through the early morning hours. Eventually, this theta-e nose will intersect the main h5 trough across western Nebraska with decreasing LIs/increasing MLCAPE reaching -4 to -8 degC/1500-2500 j/kg ranges respectively. Compressing flow aloft with the approaching h5 trough axis will yield increasing mid-level flow which in turn will strengthen deep layer shear. By sunrise Saturday, 0-6km BWD values will exceed 30 knots across the whole of the area and peak around 40 knots for the Sandhills in closer proximity to the main mid-level speed max. All this to say that ongoing activity from the northwest will encounter a more favorable environment and increase the threat for strong to severe thunderstorms in the morning hours. While questions linger about how early storms can become surface based, steep lapse rates within strong unidirectional flow from the level to MU parcel through the mid-levels supports a threat for large hail with isolated 2"+ hail possible in supercellular structures. Latest SPC Day 2 outlook highlights much of the area in a Slight Risk (level 2 of 5) and am in agreement with this placement as well as the staggering of greatest wind threat further south and east due to the previously mentioned uncertainty of if and when storms can become surface based. Renewed thunderstorm development appears probable during the afternoon along the trailing surface trough/instability gradient but expectations are for this activity to have shifted south and east of the forecast area. Thunderstorm activity should depart the area by early evening and this idea is supported by quickly decreasing SREF/HREF precipitation probabilities and raw CAM guidance. Temperatures behind convection should recover well with afternoon highs in the upper 70s to middle 80s, the coolest values expected for central Nebraska which should see a later end to thunderstorms. Saturday Night through Sunday Night...behind departing storms and after temperatures have somewhat recovered, southeasterly flow will redevelop which will help prevent boundary layer moisture from scouring out. As such, with steep lapse rates aloft remaining steadfast across our western zones, a moderately unstable atmosphere will redevelop during the afternoon. A progressive cool front will invade the area and encounter the increased instability with fairly focused, strong low-level lift. With rain and thunderstorms possible and the environment conducive for organized convection, cannot rule out another overnight threat for strong to potentially severe thunderstorms. Given the timing of the day, expecting the main threat to be elevated hail storms with a few 1" hail reports possible. With precipitable water values having likely recovered ahead of the front, am a little concerned about the prospect of locally heavy rainfall. Synoptic models, notably the GFS/ECWMF are a little more pessimistic about the potential while the CAMs are much more bullish. Generally speaking, will side with the CAM guidance with support from SREF output suggesting the potential for wetting rains in the area. Whether this winds up being a widespread issue remains in question as individual NWP solutions only show a few more isolated stronger cores versus widespread such as with a MCS. NBM 90th percentile QPF values show values nearing 0.50" for much of the area through the day Sunday. Latest QPF from the WPC shows slight increases locally and would expect to see these continue to trend upwards should confidence in solutions such as the NAM become more certain. Similar to Saturday, timing will favor Sunday morning through early afternoon so the majority of activity should exit the area by early evening. Temperatures will likely be slightly cooler in the post frontal airmass with highs remaining in the 70s for all. Northerly flow will persist into the evening hours as high pressure settles into the central Rockies. This will be the beginning of another fairly quiet period as decreasing moisture due to incoming dry air will help stymie precipitation chances for the extended period. Lows Sunday night will settle to below normal values due to light northerly winds and clearing skies. && .LONG TERM /MONDAY THROUGH FRIDAY/... Issued at 221 PM CDT Fri Jul 5 2024 Monday...perhaps the best potential for precipitation in the long term forecast prior to high pressure aloft moving in. Elongated trough axis will remain stagnant from the upper Mississippi Valley down through the central Plains. Even though the main trough axis will likely settle east of the area, enough low-level lapse rates may induce a few isolated showers across central Nebraska. Forecast soundings depict reasonable mid-level moisture and lapse rates to support isolated development, similar to what is observed today. For now, the NBM populates a dry forecast but as the day draw nearer, would not be surprised to see a need to introduce low-end PoPs to the forecast. Tuesday and beyond...long term will be characterized by increasing ridging aloft working in from the west with highly amplified northwesterly flow. This plays out typically with increasing temperatures and limited precipitation chances. Ridge axis arrives into the Great Basin by mid-week with building high pressure aloft finally reaching into the central Plains by late Thursday. While most of the anomalous heat will be focused west of the Rockies, temperatures will still likely climb through next week with near steady 1-3 degF day-over-day increases in NBM 25th, 50th, and 75th percentile values. Meanwhile after Tuesday, QPF probabilities nearly flatline with exceedance probabilities of only 0.01" falling to less than 20% each day Wednesday through Saturday. Individual ensembles such as the EPS and GEFS do not inspire much more hope with only a smattering of foundational members suggesting measureable precipitations. Signals increase somewhat by early the following week but at this range, timing is murky at best. For now, expect the dog days of summer to return by late next week with highs returning to above normal values and precipitation chances to be few and far between. && .AVIATION /00Z TAFS THROUGH 00Z SUNDAY/... Issued at 629 PM CDT Fri Jul 5 2024 VFR will continue for western and north central Nebraska terminals through tonight as mid level clouds dissipate and surface winds lighten. Scattered thunderstorms develop northwest early tomorrow and spread southeast during the day, arriving sometime around sunrise near VTN and midday near LBF. In the wake of the activity, southeast winds transition to northeast (LBF) or northwest (VTN). && .LBF WATCHES/WARNINGS/ADVISORIES... None. && $$ SHORT TERM...NMJ LONG TERM...NMJ AVIATION...Snively