textproduct: Cleveland

WHAT HAS CHANGED

The evolution of ongoing showers and thunderstorms ahead of the approaching cold front Saturday morning remains uncertain, which continues to limit confidence in how organized the local severe thunderstorm threat will be Saturday afternoon and evening.

KEY MESSAGES

1) Isolated shower/thunder chances through early Saturday, with greater coverage of showers and storms expected to develop late Saturday morning and afternoon. A few storms may turn severe, particularly between 12 PM and 5 PM east of the I-71 corridor. Potential for organized severe weather remains uncertain.

2) The next period of unsettled weather is expected Tuesday into Wednesday, with the greatest potential for rain and perhaps more thunderstorms expected Tuesday night into Wednesday.

3) Above average temperatures will continue through Tuesday night. Temperatures will return back to near or slightly below normal by late Wednesday into Thursday.

DISCUSSION

KEY MESSAGE 1... A couple subtle pushes of low-level theta-e advection are expected through early Saturday...one pushing across Northwest OH early this afternoon and expected to progress east-northeast into this evening...and another expected to push east across the area later this evening and overnight. Each may provide enough forcing for isolated shower/thunder potential at times through early Saturday, though the general flavor of the forecast will be mainly dry (and quite warm/humid by early March standards!) through the rest of today and tonight. It will be remarkably mild tonight with overnight lows ranging from the upper 50s to low 60s, outside of the chillier immediate lakeshore.

The main focus for rain and thunder potential, along with a risk for severe weather, remains during the day Saturday along and ahead of a cold front that will sweep through. However, the impact of pre-frontal convection that will likely be ongoing Saturday morning on the severe threat remains uncertain, leading to lower confidence in that severe potential.

Guidance is in good agreement that convection will be ongoing just to our west along a pre-frontal trough/outflow boundary ahead of an approaching cold front to start Saturday morning, with mainly dry (and warm) weather expected to start Saturday across the area. This activity is expected to continue east into Northwest Ohio by later Saturday morning, likely on a weakening trend through the late morning during the diurnal convective minimum. By midday/early afternoon, destabilization and gradually increasing forcing (evidenced by subtle 500mb height falls and the approaching right-entrance quadrant of a 150kt upper jet streak) will lead to convection increasing in coverage and intensity along the advancing pre-frontal trough/outflow boundary. There's also good agreement that comparatively weaker showers/storms will try to develop along the cold front itself, pushing into Northwest Ohio later Saturday afternoon and progressing east-southeast across the local area through Saturday evening. Given all of this, am generally expecting two rounds of convective potential to sweep east across the area Saturday into Saturday evening with a brief lull in between.

Where the uncertainty comes in is how quickly that first/pre- frontal round of convection moves east across the area. The more conducive thermodynamic will exist ahead of this activity, with weaker heating and somewhat weaker mid-level lapse rates behind it. If this initial activity advances through faster it will have less time to intensify and organize across our area, keeping the greater severe threat to our southeast. A number of 12z models suggest that, lowering confidence in severe potential locally. It is a fine line as convection will likely start upticking by midday, so even a slightly slower trend (as several earlier HRRR runs showed) could lead to a round of organized convection with severe potential as far west as about the I-71 corridor. The environment ahead of the pre-frontal trough is expected to feature modest (500-1000 J/KG of MLCAPE) instability, somewhat steep (~6.5C/km) mid-level lapse rates, reasonably well-mixed low-levels (sfc-3km lapse rates ~8.0C/km), and moderate effective bulk shear (~40-45kt). Quickly increasing but largely unidirectional low-level wind profiles should supply ~40kt of 0-3km shear and 150-200 m2/s2 of effective SRH. The uncapped instability, increasing forcing, and deep-layer shear largely paralleling the surface boundaries argue for a mainly messy/linear storm mode, with short line segments likely favored after what may be a brief, early window of semi-discrete activity. This adds up to potential for an organized damaging wind threat with more robust line segments, with a non-zero (but not incredibly high) tornado risk given sufficient low-level shear for mesovort development. There may be some hail, especially with any more discrete cells, though that's more of a secondary threat. Given lingering uncertainty, the Day 2 SPC Convective Outlook remains unchanged with a Slight Risk (level 2/5, driven by damaging wind potential) primarily east of the I-71 corridor...the location of ongoing convection early Saturday and how quickly it moves east across the area will be critical to the overall severe threat.

Any convection that spreads in from the west during the late afternoon/early evening along the cold front itself will be in a very marginal thermodynamic environment with weak/skinny instability profiles. However, strong forcing and deep-layer shear could allow for at least an isolated severe threat late Saturday afternoon and early evening with any cold frontal convection. The potential for stronger convection on the cold front is generally low, but could increase a bit if the first round exits quicker. A Marginal Risk (level 1/5) covers our western counties between both potential rounds of convection.

Area average QPF through Saturday evening is mainly modest, generally in the 0.10-0.40" range (highest southeast). Locally 1.00-2.00" may occur if any parts of the area see multiple storms or brief training convection, though those higher amounts would be the exception rather than the rule. Shower and storm potential exits to the southeast Saturday night with the cold front. Highs will surge into the upper 60s/lower 70s on Saturday (warmer if the initial round of convection holds off slightly longer). Lows Saturday night return to the 40s behind the front.

KEY MESSAGE 2... Dry weather is favored for Sunday and Monday. An amplifying trough is expected to approach the region on Wednesday and move through Wednesday night and Thursday. Moisture return begins on Tuesday amid deep-layer southwesterly flow ahead of the incoming trough. A developing low pressure and trailing cold front are expected to cross late Tuesday night or Wednesday. Some shower/thunder potential is possible Tuesday as warmer and more humid air returns, though the greatest coverage is likely Tuesday night into Wednesday. Trends in QPF and the timing of the front will be monitored for flooding/severe related hazards. At the least, there's decent potential for a widespread wetting/beneficial rain. Guidance currently has the front progressing through earlier in the day Wednesday which would limit any severe threat, though given that's still several days out trends in the timing of the front remain possible.

KEY MESSAGE 3... Temperatures will largely be stuck in the 50s on Sunday behind this weekend's cold front, rebounding into the 60s to perhaps low 70s for highs on Monday and Tuesday. Wednesday will likely be a transition day as the cold front moves through, with highs on Thursday near or a bit below normal, possibly staying in the 30s across parts of the area. Friday likely sees temperatures trend back up, though that appears to be short-lived with ensembles suggesting a pattern change around mid-March.

AVIATION /18Z Friday THROUGH Wednesday/

Terminals in IFR conditions still are awaiting a warm front to move north of the area, and at that time, significant improvement to the flight conditions are expected. Southerly surface flow commences in the wake of the warm front as well, and into the overnight hours, LLWS will be an issue as the surface winds get cut off from the stronger low level wind flow. All terminals will be subject to this. Take the LLWS out of the forecast with sunrise and daytime heating/boundary layer mixing occurs. Timed some convective activity for the western terminals as its arrival should be prior to 18Z, and for CLE, prior to 00Z Sunday. Cold front follows the convection, but beyond the scope of this forecast.

Outlook...As showers/storms exit Saturday, non-VFR in low ceilings expected Saturday night.

MARINE

Mobile pack ice continues to dominate Lake Erie with warmer temperatures and higher dewpoints aiding in further melting. Patchy dense fog remains an issue into this evening and possibly again during the overnight. Wave heights limited by the aforementioned mobile pack ice, which will shift around with winds increasing out of the SSW tonight into Saturday 20-35kts. Thunderstorms possible Saturday as a cold front works its way across Lake Erie later in the day. Winds become westerly 10-20kts Saturday night, returning to southwesterly Monday through Tuesday 15-25kts.

CLE WATCHES/WARNINGS/ADVISORIES

OH...None. PA...None. MARINE...Dense Fog Advisory until 7 PM EST this evening for LEZ142>146- 162>166.