Perfecto Weather ForecastPrepared for Chicago, IL

Meteorologists

Matthew Sanders

Lantao Sun

48 Hour Mesoscale Discussion for 12Z Thursday Feb 16 to 00Z Friday Feb 17

An area of low pressure over western Kansas Wednesday night will ride northeast along a stationary front to southern Michigan Thursday evening. As the cyclone traverses the Mississippi Valley it will tap into copious amounts of GOMEX moisture. This moisture will be transported out of the Gulf and into the Mississippi River Valley on a robust southwest flow. The flow combined with an h85 LLJ of 65 to 70 kts will lead to intense WAA over the cold dome of air progressing southeast from central Canada. This is depicted well on x-sections by the strong isentropic lift near h70 over the theta E field. X-sections also show that over Chicago these dynamics lead to llvl increases in the RH field by 12z Thursday. It will take until midday for dry air to be scoured out of the column above 700mb as the LLJ moves into the area from the southwest. Additionally the left front region of a h25 jet provides support during the afternoon as it progresses eastward and over the region. Thus, expecting low clouds and light intermittent precipitation in the Chicago area overnight and through early Thursday afternoon. The column saturates as dynamics become more favorable in the afternoon and this will lead to a steadier precipitation. Llvl shear and TT’s in the mid to upper 50s could lead to heavier convective precipitation by late afternoon. Intense frontogenisis and h70mb vertical velocities over the Chicago area during the afternoon support this scenario of increased precipitation by afternoon. Additionally PW vales increase from .5in during the morning to over 1in during the afternoon.

Precipitation type will initially be tricky as a very tight baroclinic zone resides along the front and this means a sharp transition from frozen to liquid precipitation. At 12z Thursday the 18Z/14 NAM indicating 0C line at the surface running through central Cook county with the 0C at h85 just to its north. The 12Z/14 WRF agrees with this scenario while the 12Z/14 GFS and ECMWF provide a slightly warmer solution. Wind profiles indicate a cool llvl NE flow at the surface and the NAM/WRF with their superior BL resolutions may have this better resolved. NAM soundings on the northwest side of the city indicate a very shallow surface layer below freezing at 12Z. On the south side and along the lakefront the surface is at or just above freezing. All locations move above freezing by mid morning as warm air mixes down. Thus, the northwest side of the city may experience some frozen precipitation during the very early morning with areas along the lake and to the south experiencing the liquid variety. On the northwest side this will likely take the form of freezing drizzle and light freezing rain. The shallow RH is below the favored snow growth location and temperatures above 900mb are over freezing so possibly just some intermittent flurries & SW mixed with IP from time to time. This could lead to a dusting or less of snow accumulation and 1/10 of an inch or less of ice. Along the lake and to the south drizzle and light rain will be the rule though there could be some residual slick spots after 12Z from the colder overnight conditions. Liquid precipitation will take over in all locations after mid morning as surface temperatures rise. Precipitation will end as a brief period of snow showers after midnight on Thursday as the lows cold front crashes southeastward and across the city.

48 Hour Forecast for 12Z Thursday Feb 16 to 00Z Friday Feb 17

Winter weather conditions will brush Chicago’s northwest side Thursday morning as a storm takes shape in the Mississippi River Valley. A weak area of low pressure over Kansas Wednesday night will intensify into a powerful storm as it taps into Gulf of Mexico moisture and moves northeastward into Michigan by Friday. The city will experience a mix of sun and clouds on Wednesday as the mercury climbs into the low 40s. Overnight temperatures will fall to freezing and some light snow and freezing rain will affect the area during the early morning hours. A warm southwesterly flow ahead of the storm will push the cold air out of the city Thursday morning. However, in the northwest part of the city the cold air may hold on long enough to affect the morning commute with some light freezing rain and freezing drizzle. Accumulations during the commute appear minimal with a dusting or less of snowfall and 1/10 of an inch or less of ice. The lakefront and southern portions of the city will be sufficiently warm so that only liquid precipitation will be falling from the sky. The entire city will be experiencing light rain by mid morning as the frozen precipitation pushes north of the area. The storm strengthens and nears Chicagoland by the afternoon so look for rainfall to intensify as the day progresses. Temperatures will top out near 36 by the afternoon and then fall sharply during the early evening as the storm’s cold front passes overhead. As the mercury falls a transition back to some light snow showers may occur by midnight before the precipitation comes to an end. Any liquid precipitation on the roadways will refreeze and create slick spots by mid evening as the temperatures tumble.

12Z/2/14 GFS 6hr Precipitation

12Z/2/14 GFS h85 Temperatures

North Chicago at 12 & 18 Z South Chicago / Lakefront at 12 & 18 Z

18Z/2/14 NAM Wind Field

18Z/2/14 NAM Wind RH Field

18Z/2/14 NAM PW

18Z/2/14 NAM Frontogenisis

18Z/2/14 NAM h70 Vertical

Velocities & TTs

24 Hour Mesoscale Discussion for 12Z Thursday Feb 16 to 00Z Friday Feb 17

Model suite continue to advertise an area of low pressure tracking from the mid Mississippi River Valley Wednesday night into the LP of Michigan Thursday evening. The NAM is slightly more progressive than the GFS that depicts a slower and more wound up system. Despite this, both models along with the ECMWF and WRF are very similar in their supporting dynamics. A trend towards a warmer solution for the city of Chicago has been occur in all these models since yesterday evening. During the Thursday morning commute the 12Z/15 NAM now shows the 0C line at the surface in the far NW corner of Cook County and moving northward. The warm air aloft results in the 0C line at h85 being north of the city at 12Z. The GFS depicts a slightly warmer solution. Previous forecast at 48hrs indicated a slight chance of frozen precipitation in the NW portion of the city. The trend to a warmer solution would lessen this threat even further. Thus, there is now only a very small chance that any precipitation falling at or just after 12Z in the NW part of the city will be frozen. The remainder of the city will certainly experience liquid precipitation during the morning commute. However, the sharp baroclinic line will reside just north of the city throughout the day and will need to be closely monitored for any southward movement. The 09Z/15 SREF support this scenario by depicting little if any chance of ZR after 12Z. The SREF also has no probability of frozen precipitation accumulating on roadways during the morning commute.

The threat of frozen precipitation during the morning commute is lessened by the fact that the column will be quite dry above h70 at that time. X-sections depict only llvl moisture capable of producing drizzle and possibly some ZR-. In fact the models have been trending towards a dryer solution since yesterday. The NAM moisture profiles continue to advertise a dryer solution than the GFS and this may be due to the NAM’s better llvl resolution. While the shallow NW surface flow may aid in holding some cold air into the NW portion of the city it is also drying out the llvls of the column. Thus, do not expect much more than drizzle and light intermittent RW through midday in the city. The afternoon, especially during the evening commute, is a different story. As depicted in the 48hr disco there remains a robust llvl jet at h85, TT’s in the upper 50s, llvl shear, PW over 1in, intense frontogenisis, and strong vertical velocities during the mid to late afternoon over the city. Additionally the left front region of a h25 jet aids its support during this time. This will result in more persistent and heavier rainfall as the afternoon progresses. Some of this precipitation is likely to be convective in nature due to the supporting dynamics so we could see some rumbles of thunder. X-sections depict the increase in afternoon moisture quite well as RH filds increase above h70 due to pronounced isentropic lift. The threat of snow showers still exists for a few hours as precipitation is exiting and the event is winding down during the evening.

24 Hour Forecast for 12Z Thursday Feb 16 to 00Z Friday Feb 17

An area of low pressure will move northeast from the mid Mississippi River Valley Wednesday evening to over the Chicago metropolitan area on Thursday. There is still a very small threat of frozen precipitation in the northwest part of Cook County near the time of the Thursday morning commute. Computer models continue to advertise a warmer solution with this storm so the probability of freezing precipitation is lessening. Nonetheless there may be a very brief period of freezing drizzle and very light freezing rain for the early portion of the morning commute in the northwest portion of the city. Otherwise the southern part of the city and along the lakefront will experience light drizzle through the morning hours. However, this situation will have to be closely monitored as cold air will be nearby and any change in the storm track could produce more frozen precipitation over the city during the morning. Temperatures will slowly rise from near the freezing mark to a high of 36F by early afternoon. As the storm taps into abundant Gulf of Mexico moisture precipitation will increase in intensity during the afternoon. The city can expect heavy rain showers, gusty winds, and possibly the rumble of thunder during the evening commute. As the storm moves northeast its associated cold front will sweep through the area during the evening. This will transition the precipitation back to some light snow showers during the mid evening as the event is winding down.

09Z/2/15 SREF Probability of ZR>.01in

06Z

18Z 12Z

09Z

09Z/2/15 SREF Probability of Accumulation on Roads

12Z 18Z

Chicago NOWCAST

A cold front currently lies just to the SE of Chicago on a line from Bloomington, IL to Valparaiso, IN. Showers and embedded thunderstorms are currently riding just behind this front and affecting the Chicago metropolitan area. As the front progresses eastward showers and thunderstorms will continue to affect the Chicago area for the next several hours Some of these thunderstorms can contain strong damaging wind and small hail.. The precipitation should lessen in intensity as the afternoon progresses so look for some moderate to light rain to be affecting the area during the evening commute. Rain showers will transition to light snow showers as the event is winding down near 9pm and temperatures quickly fall below freezing. These snow showers will persist until midnight but be light and widely scattered so no accumulation is anticipated. Strong and gusty winds should aid in evaporating liquid precipitation from the pavement so this will lessen the threat of black ice formation as temperatures fall.

At ORH at 5pm expect light to moderate rain showers with occasional embedded thunderstorms and a low cloud ceiling. Strong low level winds will result in strong shear just off the deck. IFR or LIFR flight rules are recommended due to visibilities being lowered to ½ mile or less in any heavy showers or thunderstorms.

Critique of Forecast for 12Z Thursday Feb 16 to 00Z Friday Feb 17

Thursday February 16, 2006 brought an eventful day of weather to the city of Chicago, IL. The morning dawned with misty conditions that featured low clouds and drizzle with temperatures a tick above the freezing mark. The mercury steadily rose as the day progressed, reaching a high of 36F at 18Z at ORD. Drizzle transitioned to RW/RW+ with imbedded T by late morning and these conditions persisted until 22Z when a cold front crossed the area. Behind the front temperatures tumbled, settling into the low 20s by midnight. A period of gusty winds and SW- affected the area throughout the evening as moisture wrapped around an area of low pressure heading NE through the LP of Michigan. Clearing skies ensued after midnight as high pressure from central Canada moved into the upper Midwest.

The model suite performed quite well when forecasting the 2/14/06 weather for the Chicago Metropolitan area. The GFS and NAM were the primary NWP models used for this forecast with support from the ECMWF, WRF, and SREF. The GFS verified the best from 48 hours out with an excellent handle on the track of the low pressure. The NAM was very close to the GFS but tracked the low about 45 to 65 miles too far south. Though this track error was quite small, it meant a world of difference due to the extremely sharp baroclinic zone associated with this storm. If the NAM’s track at 48 hours had verified the NW portion of Chicago would have experienced a round of freezing precipitation during the morning commute. At 48 hours the GFS also showed a more wound up system with a 1004mb low passing over the city during the afternoon. The NAM had a more progressive system with a higher central pressure. Again, the GFS verified with ORD reporting a pressure of 1004mb just as the low crossed over the city. Precipitation timing and intensity at 48 hrs were handled better by the GFS because of its more correct forecast of the synoptic situation. The onset of the heavier precipitation occurred about 2 hours before the GFS predicted it would commence but at 48hrs out this was en excellent forecast. The GFS and NAM both resolved the convective nature of the precipitation, showing TT’s in the mid to upper 50s and a strong LLVL jet and shear to promote such conditions. Finally, the GFS indicated a sharp decrease in temperatures as the low’s cold front swept across the area during the early evening. Wrap around moisture leading to SW- during the evening was more robust on the GFS.

Continued On Next Slide….

The GFS continued to tow the same line 24 hours from the event with continued support from the ECMWF. The NAM at 24 hrs was more inline with the GFS but still had the center of low pressure just a bit too far south and less intense. Both models continued to show the onset of heavier precipitation a few hours later in the afternoon than it actually occurred. However, the RH fields from the models were excellent in resolving the situation during the morning hours. X-sections showed moisture contained at very low levels, sub h70. This gave confidence to the morning forecast of low clouds, drizzle, and very light rain that verified 24 hours later. Isentropic lift increased during the afternoon on the x-sections, transporting a greater amount of moisture into the column. This supported the forecast of heavier precipitation during the afternoon that also verified despite a few hours of timing difference. Strong supporting dynamics in the form of a llvl jet, the left front region of a h25 jet, TT’s in the 50s, strong frontogenisis, and intense h70 VV’s continued to be depicted by the GFS and NAM at 24 hrs. This gave more credence to the idea of RW/RW+ and T during the afternoon. Strong showers and convection did occur as seen on KLOT and reported by numerous ASOS’s. Models continued to indicate the passing of the cold front during the early evening and a quick collapse of the baroclinic zone leading to SW-. This took place as ORD reported a sharp decline in temperatures and several hours of SW- during the evening. The tight pressure gradient and associated gusty winds depicted by the models verified right on time during the afternoon and evening. A high of 36F, forecasted 48 hours before the event, was right on the money as temperatures topped out at 37 at ORD. The SREF was quite helpful when it came to precipitation type and accumulation on roadways. At 24hrs the SREF indicated any overnight ZR- ending at or just before the forecast period beginning at 12Z Thursday. The frozen precipitation ceased a few hour prior to the SREF prediction. However, this gave confidence in forecasting little if any frozen precipitation at the start of the period. On support from the SREF the forecast also indicated that any frozen precipitation would not accumulate on the roadways. This further helped to downplay the threat of icy roadways and the need for a DPW response.

In retrospect the headlines referring to “little if any” frozen precipitation could have been removed from the forecast. However, a slight error in the model track could have produced frozen precipitation in the Chicago area at the beginning of the forecast period. In this type of situation the forecaster feels that leaving such headlines in the forecast as a “heads-up” to DPW crews was appropriate. Overall the forecast verified quite nicely. The predictions made at 48 hours were very on the money especially considering the dicey situation and the never perfect model predictions. In fact, this forecaster was surprised by the accuracy of the GFS at 48hrs considering its rather poor track record of handling systems during a pattern change like the situation on Thursday.

OBS from ORD for 2/16