Sunrise 5:26 AM   Sunset 8:09 PM Moonrise 1:26 PM   Moonset 2:32 AM

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LMZ646 Wind Point Wi To Winthrop Harbor Il- 1105 Am Cdt Thu May 16 2024
Rest of today - Southeast wind 10 to 15 knots veering south late in the afternoon. Chance of showers and Thunderstorms through the day. Waves around 1 foot. Tonight - South wind 5 to 10 knots veering west early in the morning. Chance of showers and Thunderstorms through around midnight. Patchy fog after midnight. Waves around 1 foot. Friday - Northwest wind 5 to 10 knots veering east late in the afternoon. Areas of fog in the morning. Waves around 1 foot. Friday night - Southeast wind 5 to 10 knots backing east late in the evening, then easing to 5 knots after midnight rising to 5 to 10 knots early in the morning. Partly cloudy. Waves around 1 foot.

LMZ600
No data

FXUS63 KLOT 161140 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 640 AM CDT Thu May 16 2024

KEY MESSAGES

- Scattered showers and thunderstorms will develop this afternoon. A stray strong storm cannot be ruled out near the Wisconsin border this evening (5% or lower chance).

- Patchy to areas of dense fog may develop overnight, especially northwest of I-55.

- Friday will be pleasant with highs around 80 and nearly calm winds. A lake breeze will surge inland during the afternoon, and a stray storm cannot be ruled out south of Interstate 80 (20-30% chance).

- Aside from isolated storm chances Saturday, mostly dry conditions are expected this weekend.

- An active pattern is likely to develop next week bringing several opportunities for showers and storms.

DISCUSSION
Issued at 304 AM CDT Wed May 16 2024

Through Friday...

An early-morning hand surface analysis revealed a surface low pressure system in central Minnesota along a cold front arcing southwestward through western Iowa, southeastern Nebraska, and far northwestern Kansas. A few thunderstorms were noted ahead of the front in western Wisconsin and central Iowa. A second area of low pressure (really a mesoscale convective vortex, or MCV) was noted along the Oklahoma and Kansas Border along the backside of a relative disorganized mesoscale convective system approaching western Missouri. Ahead of the MCV, separate areas of showers and thunderstorms, and cold front, an expansive area of low-level moisture with surface dew points in the low to mid 60s and PWATs ranging from 1.0 to 1.3" (re: 00Z TOP and SGF RAOBs). The northern edge of the low-level moisture remains stark and near the Illinois and Missouri border, thanks to continued, albeit light, northeasterly flow on the backside of a departing high pressure system moving into southern Ontario. Finally, a baggy upper-level trough is evident across the Upper Mississippi River Valley water vapor imagery, including a subtle embedded shortwave propagating into northwestern Iowa at press time.

The forecast for today remains on track. Pressure falls across the northern Great Lakes will cause surface winds to turn southwesterly this morning, allowing the reservoir of moisture to our southwest to creep northeastward. Forecast soundings depict minimal capping by early afternoon which should allow for widely scattered showers and a few thunderstorms to develop in somewhat random fashion. However, heavy cloud cover should stunt MLCAPE from growing to more than 750 thereby limiting the intensity of any thunderstorm that develops.
With that said, any clearing that can take place late this afternoon particularly near the Wisconsin border may allow for locally higher instability and steeper low-level lapse rates ahead of the approaching cold front. As a result, we can't rule out a locally strong thunderstorm from developing along the cold front this evening between 4 and 8 PM, again, near the Wisconsin state line (generally a 5% chance or lower). Overall, the concern for severe weather today is pretty darn low.

Tonight, the cold front will decay into a surface ridge directly overhead, leading to nearly calm winds. Thinning cloud cover, especially northwest of I-55, will allow for efficient radiational cooling within the still-moist airmass, supporting areas of fog.
Conceptually, it's easy to envision pockets of dense fog (with visibility less than one quarter of a mile) developing by daybreak, particularly in areas where wet vegetation remains.

Tomorrow, any morning fog will erode and give way to partly cloudy skies nearly calm winds and highs near 80 will make for a spectacular and warm May day. The exception, of course, will be along the Lake Michigan shoreline as a lake breeze surges inland during the afternoon hours. Meanwhile, south of I-80, a few showers or thunderstorms cannot be ruled out as a weak wave passes overhead. Perhaps the best chances, generally between 20 and 30%, will be along any intersection of the southward-moving lake breeze and any northward-moving outflow from thunderstorms across central Illinois and Indiana.

Friday Night and Beyond...

The upper trough progged to bring us rain chances on Friday is expected to be traversing along the southern Ohio River Valley as we head into Saturday. Guidance has come into notably better agreement that the main core of the trough should be well east of our area by Saturday morning allowing mid-level height rises (higher pressure) to build into our area. However, forecast soundings do show some modest instability developing across the southeastern corner of our forecast area (i.e. areas south of the Kankakee River and east of I-57) which in conjunction to the closer proximity to the trough may allow for a low chance (<20%) of an isolated shower and/or thunderstorm Saturday afternoon. Additionally, guidance continues to depict a northern stream shortwave moving across the Upper Midwest toward the northern Great Lakes which is expected to push a cold front through northern IL Saturday night. Since the anticipated arrival time of the front is expected to be after sunset in our area, guidance continues to favor any showers/storms along the front dissipating prior to reaching the I-39 corridor. Thus, I saw no need to change the dry POPs offered by the NBM but still caution that a stray shower or rumble of thunder near I-39 and the IL-WI line Saturday evening is not completely out of the question just yet.

Aside from these minor rain chances, dry conditions are largely expected for the weekend as the aforementioned height rises pivot overhead on Sunday. Despite the passage of the cold front Saturday night, temperatures for both Saturday and Sunday are expected to be in the seasonably warm category with highs in the lower to mid-80s and overnight lows in the 60s. Though, the light surface winds will allow daily lake breezes to develop and surge inland keeping high temperatures cooler (in the upper 60s to lower 70s) for locales near the lake.

Heading into Monday and the beginning of next week, guidance continues to be in strong agreement that a more active pattern is likely to develop across the central CONUS. While there are some subtle differences in timing and intensity, the general idea is as follows. The northern jet stream is forecast to dig into the Pacific Northwest and phase with a developing sub- tropical jet across the southwestern US on Sunday. This phasing is expected to develop a broad trough across the western CONUS on Monday allowing the aforementioned sub-tropical jet to intensify and nose into the southern Plains and mid-Mississippi Valley. Not only will the sub-tropical jet aid in enhancing the southwest low-level flow over the Plains and Midwest supporting seasonably warm and humid conditions, but it will also allow any shortwaves that break off to eject towards the Midwest and southern Great Lakes. This combination of synoptic forcing, warm-humid air, and potential wind shear should promote the necessary ingredients needed for period of rather stormy weather in the region next week including the threat for severe weather.

As stated above, there are some subtle timing issues as to when any shortwaves will break off and where exactly they will track. However, the 00z guidance suite has started to highlight a couple periods of potential interest for our area. The first of these periods is within the Monday through Wednesday timeframe with the second favored toward the end of next week into the following weekend. Obviously the coverage and severity of any potential storms in our area remains uncertain given we are still 5-7+ days out, but this will be a period to monitor closely. Therefore, we continue to recommend keeping an eye on subsequent forecasts over the coming days for any changes. It may also be wise to review your severe weather plans just in case our area does become favored.

Yack

AVIATION /12Z TAFS THROUGH 18Z FRIDAY/
Issued at 640 AM CDT Thu May 16 2024

Forecast concerns for the terminals...

- Scattered showers and thunderstorms this afternoon through early evening

- Potential for patchy fog to develop late tonight into Friday morning, with a growing signal for dense fog in spots

- Weak lake breeze to push through the Chicago terminals early Friday afternoon

VFR cirrus continues to stream into the terminals this morning ahead of an upper-level disturbance pivoting through the Upper Midwest. As this disturbance gets closer to northern IL later this morning it will force winds to settle into a southwesterly direction and therefore aid in increasing low-level moisture.
This moisture increase will in turn allow instability to build and allow scattered showers and thunderstorms to develop around midday and persist into the early evening hours. While widespread severe weather is not expected, an isolated storm or two could become strong along the IL-WI line this afternoon and produce localized instances of gusty winds and/or small hail.

As showers and storms come to an end this evening (by 03-04z)
winds will briefly become southerly before veering towards a west-northwest direction overnight. Though, speeds are expected to remain light around 5 kts. These prevailing southerly winds will also aid in keeping low-level moisture elevated through the night and allow for the development of patchy fog after midnight (even at ORD and MDW). Guidance has continued to be in good agreement on timing and widespread coverage of fog, but continues to vary on the fog's density. That said, there is a growing signal that IFR visibilities are possible (around 30% chance) but given the lower confidence have decided to go with MVFR visibilities (4-5 SM) in the TAFs for now. Regardless, the fog is expected to dissipate within an hour or two after sunrise Friday morning leaving VFR conditions through the rest of the period.

Finally, a weak lake breeze is expected to develop early Friday afternoon as temperatures warm. The aforementioned light west- northwest winds look to offer little (if any) resistance and should allow the lake breeze to surge inland across the Chicago terminals between 16-18z Friday. In the wake of the breeze wind directions will become easterly with speeds increasing into the 6 to 8 kt range where they look to remain through Friday afternoon.

Yack

LOT WATCHES/WARNINGS/ADVISORIES
IL...None.
IN...None.
LM...None.

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