473 FXUS63 KIND 130708 AFDIND Area Forecast Discussion National Weather Service Indianapolis IN 308 AM EDT Wed Oct 13 2021 .Short Term...(Today through Thursday) Issued at 308 AM EDT Wed Oct 13 2021 High clouds are spreading into the Ohio Valley early this morning as moisture advects north into the region ahead of the deep surface low currently lifting through western Nebraska early this morning. Patchy fog has developed across the region as well. 0630Z temperatures were in the 50s. The initial focus for the next several hours is on further fog development and expansion. Already...seeing locally dense fog in spots with visibilities as low as 1/4 to 1/2SM in spots. With light and variable flow expected through daybreak and model soundings favoring a sharpening of a shallow inversion in the near surface layer...anticipate that fog will expand in coverage through daybreak. The big question will be how widespread visibilities lower than a mile can get...and what if any impact the high clouds streaming into the region might have at minimizing fog coverage and intensity. For now...will continue with a patchy dense fog mention in the forecast and will likely introduce an SPS as well. But have no plans at this time to introduce a dense fog advisory. Will continue to monitor through the next several hours. The strong upper level low lifting out of the central Rockies with the aforementioned surface wave will be the primary players in the forecast evolution and while neither feature will have any substantial impact on weather across central Indiana through tonight...they each will aid in setting the table for increasingly unsettled weather by late week as the trailing cold front tracks slowly across the Ohio Valley with a secondary surface wave. More on that in the long term discussion below. Over the next 36 hours or so...the upper low will lift N/NE through the northern Plains and into the eastern Canadian Prairies...eventually forcing the ridge aloft currently across the region east by Thursday. The deep surface low will followed the upper low and begin the occlusion process over Manitoba on Thursday. The trailing cold front will shift east from its position early this morning over the central Plains to the western Great Lakes and mid Mississippi by Thursday morning. From that point...forward progression will be slowed considerably as the front gets left behind by the surface low lifting into Canada and is largely blocked by a high pressure ridge sitting along the Appalachians while also becoming parallel to broad southwest flow aloft. The development of a secondary surface low along the front in the southern Plains Thursday afternoon will aid in getting the boundary moving east again...albeit sluggishly...and this will provide the expected widespread rainfall for the end of the week. For today though...the upper ridge and high pressure off to the east will largely keep conditions dry through early afternoon across the forecast area. A warm front will track into the region during the afternoon and with an axis of isentropic lift accompanying the boundary...may be just enough to generate a few showers and perhaps a couple rumbles of thunder...especially across the western half of the forecast area. The warm front will shift east this evening with southerly flow and broad warm advection developing. The bulk of the precipitation though should refocus back to the west of the region closer to the cold front and low level jet. Showers may slip into northwest counties predawn Thursday otherwise dry conditions are expected through daybreak Thursday. The surface ridge will remain close enough to be an influence on Thursday as the axis of deeper moisture remains confined close to the boundary and low level jet to our northwest. The boundary will begin to slowly shift east Thursday afternoon with chances for rain steadily increasing across the northern Wabash Valley by late day with the bulk of the widespread rainfall holding off until Thursday night and Friday. Temps...potential for fog and low clouds this morning and increasing clouds and a few showers this afternoon will limit temp rises. Low level thermals support low to mid 70s. For Thursday...increasing southerly flow and possibility for some sunshine early in the day will enable highs to rise into the upper 70s and lower 80s. Lows tonight will be much warmer with the passage of the warm front. Expect lows in the mid 60s over much of the forecast area. && .Long Term...(Thursday Night through Tuesday) Issued at 308 AM EDT Wed Oct 13 2021 The long range period will include a rainy end to the work week before a quick transition to several days of dry and seasonably cool conditions. No major changes to the high-POP, generally modest-QPF showers Thursday night and Friday morning amid the stagnant broad/ deep upper trough over the north-central US and nearly-stationary cold front to the west of central Indiana. PM hours Friday continue to should the greatest chance of heavier rains, when some of the 1.40-1.70 inch PWATs' potential should be realized as an embedded vort pulls the front across the CWA. At least moderate confidence on a sharp frontal zone quickly following, with dewpoints dropping by 20 degrees to start the weekend. Temperatures will be above normal prior to the frontal passage. Saturday through Monday...will feature broad, yet not overly potent, high pressure extended from the southern Plains to the Deep South, cutting off Gulf moisture, while upper-level northwest flow over Indiana...between the slowly departing/approaching H500 trough/ ridge...will promote subsidence and mainly clear skies. After a distinctively chilly Saturday, where brisk northwest breezes on the heels of the departing system will hold highs to near 60F...there is decent confidence in a slight moderation to near-normal readings courtesy of the generally west-southwesterly flow out of the high to the south of the region. Ensembles are indicating a reimbursing shot of cool air will try to cross Indiana midweek, although appears any window of adequate moisture the cold front will have to work with will be brief, so no more than scattered clouds included in the grids at this time. && .Aviation...(06Z TAF Issuance) Issued at 1129 PM EDT Tue Oct 12 2021 IMPACTS: - VFR Conditions may deteriorate to MVFR or IFR due to Diurnal Fog overnight. - VFR conditions are expected after 15Z Wed. DISCUSSION: GOES16 night fog product is beginning to show fog development across Central Indiana. Dew point depressions across the area show widespread areas of 3F or less. Negative crossover temps along with an approaching warm front on the backside of the high along with recent rains will result in some fog formation overnight...particularly at the more rural TAF sites. Dense fog with visibilities less than 1/2 of a mile are not out of the question. Thus have included a tempo window for IFR fog at the diurnal minimum. Any morning fog will be quick to burn off in the morning as heating and mixing resumes allowing a return to VFR by afternoon. Mid and high cloud are expected to return on Wednesday afternoon ahead of a warm front arriving on Wednesday. && .IND WATCHES/WARNINGS/ADVISORIES...None. && $$ Short Term...Ryan Long Term...AM Aviation...Puma