Upon further consideration of the setting, I'm kind of thinking that it may be a bit more like northwestern Utah or southern Idaho than central Nevada. I was also considering why the party took a full three months off during last game-year for winter. So, upon some reflection, I've reached a conclusive notion of why travelling, adventuring, or conducting military operations in the northern part of the Shieldlands in the winter is a miserable idea.
First off, orography. I'm bad at it, but the basic structure of the orography of the region that I'm considering is as follows. Warm water from the southern end of the Inner Sea flows north up the eastern coastline of that sea, nourishing the Urdukhar and Sorosi before reaching the coastal mountains of Talas. North of Soros and off the coast of Myrmidia, the waters cool. Some flow into the Bay of Seals where they are greatly chilled, while the remainder of these cold waters flow back south along the coast of the Shieldlands. As a result, the wind coming off the sea into the Shieldlands is cold and dry, carrying little water inland and explaining the relative scarcity of water in the region.
Since the humidity is low, there is rarely cloud cover, and during the summer these cooling breezes are a blessing to coastal areas. During the winter, though, they howl across the land, whipping up frigid sandstorms which chill to the bone. Worse still is when the temperature drops below freezing; mixed storms of sand and ice particles ensue, which are intensely unpleasant. Snow is as rare in the Shieldlands as rain, but waking up to find that the dew has frozen to ice is not unusual in the winter. Sometimes multiple layers of dew-ice form over the course of many days, creating a slippery crust over the sand and rocks which makes walking difficult at best and truly perilous in the highlands. Further, keeping warm is exceptionally hard, since there is a dearth of long-burning natural fuels like wood; dead, dry scrub burns too quickly for a good fire, so dried animal manure (and where available in the lowland marshes, peat moss) are the fire sources of choice for Shieldlanders. These are resources carefully gathered before winter sets in, because while it may smell bad, it beats freezing to death.
The final particular difficulty in winter in the Shieldlands is water. Always hard to come by and fiercely protected, drinking water sources like oases and creeks often freeze during the winter. This further exacerbates the problem of lack of fuel, since fire is necessary even to obtain drinkable water in many areas. As a result, towns and other settlements are typically established where there is a non-freezing aquifer which can be tapped by wells, because otherwise it is difficult to sustain large populations through winter. The more common solution in rural areas is to ferment excess foodcrops into moonshine, crude beer, and other rustic alcohols. The addition of alcohol lowers the freezing point and permits the safe, long-term storage of water. Alcohol can also serve as a food-substitute, though food is relatively easy to store safely compared to water. The necessity to store feed for livestock is likewise inconvenient for herders, but does not require unusual amounts of care. Food and animal feed are, however, particular problems for travellers during the winter, since most of the Shieldlands' already scrubby plant matter dies off during the winter.
So, winter in the Shieldlands - no food, no feed, scarce liquid water, no fuel for fire, a crust of ice on most surfaces, and cold, blowing wind carrying sand and ice with it. It's bad enough that even the orcs retire to huddle in their caves, tell tales, drink, wrestle, and breed all winter. In the early winter, they sometimes make last raids against farmers who have been storing up food, and in the late winter those clans which have run out again begin raiding to fill their bellies, but not even the orcs are foolhardy enough to raid in mid-winter.
This is becoming somewhat interesting to my PCs, because while it is currently mid-summer, they are gearing up for a military campaign against the witches who have of late taken residence in the Bleak Academy. The Academy is in the upcountry, a region of mesas and box canyons which will be tricky to bring an army through even in good conditions, and I expect that it may take them a month or more to gather an army sufficient for the operation which they wish to undertake. Then they have only fall to wage the campaign before winter sets in, and whether or not they will be able to achieve a decisive victory in those few months is uncertain. Should they fail to dislodge the witches and crush their armies, then the question of fighting during the winter becomes a very relevant one. While it is unlikely for either side to be able to continue conventional warfare through the winter, both sides do possess a few high-mobility units. On the PC side, the wizard Carcophan used a scroll of permanence to grant himself permanent flight, and he can cover in a day a distance which would take an army five or six to march. Thus, he is unencumbered by the same logistic constraints, and with his newly-acquired ability to lob fireballs, may be able to conduct aerial bombardment of beastman positions into the winter. Likewise, the party is in possession of one broom of flying, stolen from the witches themselves. While it is owned by Clovis the Clever, a thief henchman of Sir Jarol the Thothite, it can carry two riders and so could be used to deliver more fire from the sky throughout the winter. The witches, however, have already exhibited their own arcane mobility, both in creating the broom of flying in the first place and in somehow locating, suborning, and arming the elephantmen of the marshes against Carcophan's crocodilefolk.
Thus, should the war continue into the winter, it will become something of a wizard-war, with the casters of both sides arrayed against more conventional defending units. Employment in this capacity, however, denies the party's mages the usual benefit of winter, which is time to sit back, copy spells, perform research, craft items with the monster part backlog of the preceding nine months, and so forth without those thrice-damned fighters knocking on their doors for help blowing up some monster or retrieving a few shiny coins. This, as well as the difficulty of holding conquered territory in harsh weather, are factors that the PCs may do well to take into account in their strategy.