Winter Storm Forecast with a Warm Nose and Cold Roads
At 0600 UTC a winter storm is forecast for a city. The 1200 UTC sounding shows surface temperature -1.5°C and dew point -4.0°C; a warm layer from 900 to 800 hPa peaks at +2.8°C; above 800 hPa the profile is below freezing, and the surface-based cold layer is 450 m deep. The 850 hPa temperature is -2.0°C. Model QPF is 22 mm liquid equivalent in 12 hours. The ensemble gives a 45% chance of at least 15 cm of snow, a 30% chance of 5–15 cm, and a 25% chance of less than 5 cm with ice. Road temperatures are -6°C at onset but forecast to rise to -1°C. The school district must decide by 0400 whether to close schools; the highway department must decide whether to pretreat with brine or wait.
1. Determine the dominant precipitation type and the main hazard, showing how the warm-layer and cold-layer depths support your answer.
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Layer analysis. The warm layer from 900 to 800 hPa is about 1,000 m thick and peaks at +2.8°C, deep enough to melt most snowflakes. The surface cold layer is only about 450 m thick, with temperatures near -1.5°C and road temperatures of -6°C. Partial melting followed by limited refreezing favors sleet; if drops remain liquid, they freeze on contact with the cold road, producing freezing rain. Snow is unlikely to dominate.
Hazard. The 22 mm liquid equivalent is a significant ice load even if snowfall is low. The ensemble's 45% chance of at least 15 cm of snow is not the main issue; the 25% chance of less than 5 cm with ice is the dangerous branch. The forecast should emphasize ice, not just accumulation. The warm-layer depth and surface wet-bulb temperature are the key uncertainties; a slight cooling would increase snow, while a slight warming would increase rain and reduce ice.
2. Recommend a decision for the school district and the highway department at 0400, explaining how uncertainty and timing affect the choice.
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At 0400 the district must decide with imperfect information. A 45% chance of at least 15 cm is below a typical 50% closure threshold, but the ice scenario and road temperatures of -6°C make bus transport hazardous. The decision should be based on impacts, timing, and the ability to reverse a decision. A delay or closure is defensible if ice arrives before the morning commute; if the storm is forecast to arrive after 0900, a delay may be better. The district should coordinate with the highway department, use road sensors, and communicate uncertainty rather than presenting a single deterministic forecast. The key is not the snow total alone but the probability of untreated ice on bus routes.
For the highway department, pretreatment brine is most effective before the pavement is cold and before rain washes it away. With road temperatures of -6°C and a likely ice transition, brine may dilute or freeze if applied too early. A better plan is to treat priority routes at onset and hold crews for freezing rain, rather than pretreating all roads at 0400.
3. What 0200 observations would most change the forecast and the decision, and how would you use them?
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A 0200 special sounding would show whether the warm layer has eroded or strengthened. Surface temperature, dew point, and present weather (rain, freezing rain, sleet, snow) from observers and disdrometers would identify the actual transition. Road sensors and camera images would show whether ice is forming. Radar reflectivity and dual-polarization hydrometeor classification could show the melting layer. If the warm layer weakens and surface temperature falls, shift to snow and raise accumulations; if the surface warms above 0°C, reduce ice risk. If freezing rain is occurring, pretreatment may be ineffective and the plan should shift to closure and salt or abrasive application.